Interferometer having a dynamic beam steering assembly

ABSTRACT

The invention features an interferometry system which includes at least one dynamic beam steering assembly for redirecting one or more beams within the interferometry system in response to changes in the angular orientation or position of the measurement object. A control circuit controls the beam steering assembly based on a signal derived from one or more beams within the interferometry system. For example, the control circuit can cause the beam steering assembly to redirect a measurement beam within the system such that exit measurement and reference beams used to generate an interferometric signal remain substantially parallel to one another over a range of angular orientations or translations of the measurement object. The interferometry systems can be advantageously incorporated into lithography systems used to fabricate integrated circuits and other semiconducting devices and beam writing systems used to fabricate lithography masks.

BACKGROUND OF THE INVENTION

This invention relates to interferometers, e.g., displacement measuringinterferometers for measuring displacements of a measurement object suchas a mask stage or a wafer stage in a lithography scanner or steppersystem.

Displacement measuring interferometers monitor changes in the positionof a measurement object relative to a reference object based on anoptical interference signal. The interferometer generates the opticalinterference signal by overlapping and interfering a measurement beamreflected from the measurement object with a reference beam reflectedfrom the reference object.

In many applications, the measurement and reference beams haveorthogonal polarizations and different frequencies. The differentfrequencies can be produced, for example, by laser Zeeman splitting, byacousto-optic modulation, or internal to the laser using birefringentelements or the like. The orthogonal polarizations allow a polarizingbeam splitter to direct the measurement and reference beams to themeasurement and reference objects, respectively, and combine thereflected measurement and reference beams to form overlapping exitmeasurement and reference beams. The overlapping exit beams form anoutput beam, which subsequently passes through a polarizer. Thepolarizer mixes polarizations of the exit measurement and referencebeams to form a mixed beam. Components of the exit measurement andreference beams in the mixed beam interfere with one another so that theintensity of the mixed beam varies with the relative phase of the exitmeasurement and reference beams. A detector measures the time-dependentintensity of the mixed beam and generates an electrical interferencesignal proportional to that intensity. Because the measurement andreference beams have different frequencies, the electrical interferencesignal includes a “heterodyne” signal having a beat frequency equal tothe difference between the frequencies of the exit measurement andreference beams. If the lengths of the measurement and reference pathsare changing relative to one another, e.g., by translating a stage thatincludes the measurement object, the measured beat frequency includes aDoppler shift equal to 2νp/λ, where ν is the relative velocity of themeasurement and reference objects, λ is the average wavelength of themeasurement and reference beams, and p is the number of passes to thereference and measurement objects. Changes in the relative position ofthe measurement object correspond to changes in the phase of themeasured interference signal, with a 2π phase change equal to a distancechange of λ/(2p).

Such interferometers are often crucial components of scanner systems andstepper systems used in lithography to produce integrated circuits onsemiconductor wafers. Such lithography systems typically include atranslatable stage to support and fix the wafer, focusing optics used todirect. a radiation beam onto the wafer, a scanner or stepper system fortranslating the stage relative to the exposure beam, and one or moreinterferometers. Each interferometer directs a measurement beam to, andreceives a reflected measurement beam from, a plane mirror attached tothe stage. Each interferometer interferes its reflected measurementbeams with its reference beam, and collectively the interferometersaccurately measure changes in the position of the stage relative to theradiation beam. The interferometers enable the lithography system toprecisely control which regions of the wafer are exposed to theradiation beam.

In many lithography systems and other applications, the measurementobject includes one or more plane mirrors to reflect the measurementbeam from each interferometer. Small changes in the angular orientationof the measurement object, e.g., pitch and yaw of a stage, can alter thedirection of each measurement beam reflected from the plane mirrors. Ifleft uncompensated, the altered measurement beams reduce the overlap ofthe exit measurement and reference beams in each correspondinginterferometer. Furthermore, these exit measurement and reference beamswill not be propagating parallel to one another nor will their wavefronts be aligned when forming the mixed beam. As a result, theinterference between the exit measurement and reference beams will varyacross the transverse profile of the mixed beam, thereby corrupting theinterference information encoded in the optical intensity measured bythe detector.

To address this problem, many conventional interferometers include aretroreflector that redirects the measurement beam back to the planemirror so that the measurement beam “double passes” the path between theinterferometer and the measurement object. The presence of theretroreflector insures that the direction of the exit measurement isinsensitive to changes in the angular orientation of the measurementobject. However, even with the retroreflector, the lateral position ofthe exit measurement beam remains sensitive to changes in the angularorientation of the measurement object. Furthermore, the path of themeasurement beam through optics within the interferometer also remainssensitive to changes in the angular orientation of the measurementobject.

In addition, translations of the measurement object can also impartchanges to the lateral position and path of the measurement beam. Forexample, in applications in which the measurement object includes amounted cube prism retroreflector to reflect the measurement beam, atranslation of the measurement object transverse to the measurement beamimparts a lateral displacement to the reflected measurement beam that istwice the translation of the measurement object.

SUMMARY OF THE INVENTION

The invention features an interferometry system which includes at leastone dynamic beam steering assembly for redirecting one or more beamswithin the interferometry system in response to changes in the angularorientation or position of the measurement object.

In general, in one aspect, the invention features an interferometrysystem for measuring changes in distance to a measurement object. Thesystem includes an interferometer which during operation directs areference beam along a reference path and a measurement beam along ameasurement path contacting the measurement object and combines thereference and measurement beams to produce overlapping exit referenceand measurement beams. The overlapping exit reference and measurementbeams are indicative of changes in a relative optical path lengthbetween the reference and measurement paths. The interferometer includesa beam positioning system to orient the beam steering element. The beamsteering element is positioned to direct at least one of the referenceand measurement beams and the at least one of the reference andmeasurement beams contact the beam steering element. The system alsoincludes a control circuit which during operation causes the positioningsystem to reorient the beam steering element in response to changes inat least one of angular orientation and position of the measurementobject.

The system can include any of the following features.

The interferometer can separate an input beam into a pair of spatiallyseparated beams to define the reference and measurement beams.Alternatively, during operation the interferometer receives a pair ofspatially separated input beams to define the reference and measurementbeams.

The interferometer can be a single-pass interferometer in which themeasurement beam contacts the measurement object only a single time.Also, in some embodiments, the interferometer can include themeasurement object, and in other embodiments, the measurement object canbe separate from the interferometer. During operation the controlcircuit can cause the positioning system to reorient the beam steeringelement based on a signal generated from the exit measurement beam.

Also, during operation, the control circuit can cause the positioningsystem to reorient the beam steering element to do any of thefollowing: 1) maintain the exit reference and measurement beamssubstantially parallel to one another over a range of angularorientations of the measurement object; 2) reduce changes in directionof the exit measurement beam caused by changes in the angularorientation of the measurement object; 3) reduce a transversedisplacement between the exit reference and measurement beams caused bychanges in the angular orientation of the measurement object or atranslation of the measurement object; 4) reduce a transversedisplacement of the measurement beam caused by changes in the angularorientation of the measurement object or a translation of themeasurement object; and 5) insure that the measurement beam contacts themeasurement object at substantially normal incidence over a range ofangular orientations of the measurement object.

In embodiments in which the measurement object is a plane mirror andduring operation the measurement beam contacts the measurement object atnon-normal incidence, the control circuit can cause the positioningsystem to reorient the beam steering element to reduce transversedisplacements between the exit reference and measurement beams caused byby translations of the measurement object.

In some embodiments, the beam steering element can include a polarizingbeam splitter that separates the input beam into the reference andmeasurement beams, and the positioning system can include at least onetransducer mounted to the polarizing beam splitter and operative toadjust an orientation of the polarizing beam splitter in response to acontrol signal from the control circuit. Alternatively, the beamsteering element can include a polarizing beam splitter that directs thereference beam along the reference path and the measurement beam alongthe measurement path, and the positioning system can include at leastone transducer mounted to the polarizing beam splitter and operative toadjust an orientation of the polarizing beam splitter in response to acontrol signal from the control circuit.

In other embodiments, the interferometer can further include apolarizing beam splitter that separates the input beam into thereference and measurement beams, and wherein the beam steering elementincludes a beam steering mirror and the positioning system includes atleast one transducer mounted to the beam steering mirror and operativeto adjust an orientation of the beam steering mirror in response to acontrol signal from the control circuit. Alternatively, theinterferometer can further include a polarizing beam splitter thatdirects the reference beam along the reference path and the measurementbeam along the measurement path, and wherein the beam steering elementincludes a beam steering mirror and the positioning system includes atleast one transducer mounted to the beam steering mirror and operativeto adjust an orientation of the beam steering mirror in response to acontrol signal from the control circuit.

The system can further include a polarizer which during operationreceives the exit reference and measurement beams and mixespolarizations of the exit reference and measurement beams to produce amixed beam having a phase indicative of the changes in the optical pathlength difference between the reference and measurement paths. Thecontrol circuit can include a detector having spatially resolveddetector elements operative to measure directions and/or positions ofthe exit reference and measurement beams and generate a measurementsignal indicative of the directions and/or positions. The controlcircuit can further include a controller operative to receive themeasurement signal from the detector and send a control signal based onthe measurement signal to the positioning system, the control signalcausing the positioning system to reorient the beam steering element.

In addition to a polarizing beam splitter, beam steering mirror, andtransducers mounted to the beam steering mirror, the interferometer canfurther include a measurement retroreflector, a referenceretroreflector, and a reflective reference object. During operation, thepolarizing beam splitter directs the reference beam to the referenceobject and the measurement beam to the measurement retroreflector,receives the reference beam from the reference object and themeasurement beam from the measurement retroreflector, and then directsthe reference and measurement beams to the beam steering mirror. Thebeam steering mirror directs the reference beam to the referenceretroreflector and the measurement beam to the measurement object,receives the reference beam from the reference retroreflector and themeasurement beam from the measurement object, and then directs thereference and measurement beams back to the polarizing beam splitter.The polarizing beam splitter combines the reference and measurementbeams to produce the overlapping exit reference and measurement beams.The measurement retroreflector can have an aperture through which thereference beam travels to and from the reference object and thereference retroreflector can have an aperture through which themeasurement beam travels to and from the measurement object. Theinterferometer can further include a reference quarter wave platethrough which the reference beam travels to and from the referenceobject and a measurement quarter wave plate through which themeasurement beam travels to and from the measurement object.

The beam steering mirror can have front and back faces. During operationthe front face of the beam steering mirror can direct the measurementbeam to the measurement object and the back face of the beam steeringmirror can receive the measurement beam from the measurement object. Theinterferometer can further include a second polarizing beam splitterwhich during operation receives the reference beam from the referencepath and the measurement beam from the back face of the beam steeringmirror and produces the exit reference and measurement beams.

The interferometer can further include at least one additional beamsteering assembly including an additional beam steering element and anadditional positioning system to orient the additional beam steeringelement. During operation the additional beam steering element directsthe measurement beam. The measurement beam contacts the additional beamsteering element and during operation the control circuit causes thepositioning systems to reorient the beam steering elements to optimizethe overlap of the exit reference and measurement beams and maintain theexit reference and measurement beams substantially parallel over a rangeof angular orientations and positions of the measurement object. Thecontrol circuit can include two detectors having spatially resolveddetector elements operative to measure a position and direction of theexit measurement beam and generate measurement signals indicative of theposition and direction. The control circuit can further include acontroller operative to receive the measurement signals from thedetectors and send control signals to the positioning systems based onthe signals from the detectors, the control signals causing thepositioning systems to reorient the beam steering elements.

In general, in another aspect, the invention features an interferometrysystem for measuring changes in distance to a measurement object. Thesystem includes an interferometer which during operation receives one ormore input beams to define a reference beam and a measurement beam,directs the reference beam along a reference path and the measurementbeam along a measurement path contacting the measurement object, andcombines the reference and measurement beams to produce an output beamincluding overlapping exit reference and measurement beams. The outputbeam is indicative of changes in a relative optical path length betweenthe reference and measurement paths. The system further includes a beamsteering assembly including a beam steering element and a positioningsystem to orient the beam steering element. During operation the beamsteering element directs the input and output beams, the input andoutput beams contacting the beam steering element. The system furtherincludes a control circuit which during operation causes the positioningsystem to reorient the beam steering element in response to changes inat least one of angular orientation and position of the measurementobject.

The system can include any of the following features.

The interferometer can receive a single input beam and separate thesingle input beam into a pair of spatially separated beams to define thereference and measurement beams. Alternatively, the interferometer canreceive a pair of spatially separated input beams to define thereference and measurement beams.

The beam steering element can include a beam steering mirror, and thepositioning system can include at least one transducer mounted to thebeam steering mirror and operative to adjust an orientation of the beamsteering mirror in response to a control signal from the controlcircuit. The beam steering mirror can include front and back faces, andwherein during operation the input beam contacts the front face of thebeam steering mirror and the output beam contacts the back face of thebeam steering mirror. The interferometer can be a single-passinterferometer in which the measurement beam contacts the measurementobject only a single time.

The control circuit can cause the positioning system to reorient thebeam steering element to do any of the following: 1) maintain the exitreference and measurement beams substantially parallel to one anotherover a range of angular orientations of the measurement object; 2)reduce changes in direction of the exit measurement beam caused bychanges in the angular orientation of the measurement object; 3) toreduce a transverse displacement between the exit reference andmeasurement beams caused by changes in angular orientation of themeasurement object or a translation of the measurement object; 4) reducea transverse displacement of the measurement beam caused by changes inangular orientation of the measurement object or a translation of themeasurement object; 5) to insure that the measurement beam contacts themeasurement object at substantially normal incidence over a range ofangular orientations of the measurement object.

In general, in a further aspect, the invention features aninterferometry system including an interferometer which during operationdirects a reference beam along a reference path and a measurement beamalong a measurement path contacting a measurement object at non-normalincidence, and combines the reference and measurement beams to formoverlapping exit reference and measurement beams. The overlapping exitmeasurement and reference beams are indicative of changes in a relativeoptical path length between the reference and measurement paths. Theinterferometer includes a beam steering assembly positioned to directthe measurement beam. The system includes a control circuit which duringoperation causes the beam steering assembly to redirect the measurementbeam to reduce transverse displacements between the exit reference andmeasurement beams caused by translations of the measurement object.

In general, in another aspect, the invention features an interferometrysystem including an interferometer which during operation directs areference beam along a reference path and a measurement beam along ameasurement path contacting a measurement object, and combines thereference and measurement beams to form overlapping exit reference andmeasurement beams. The overlapping exit measurement and reference beamsare indicative of changes in a relative optical path length between thereference and measurement paths. The interferometer includes a beamsteering assembly positioned to direct at least one of the reference andmeasurement beams.

The system further includes a control circuit which during operationcauses the beam steering assembly to redirect one of the reference andmeasurement beams to do at least one of the following: 1) maintain theexit reference and measurement beams substantially parallel to oneanother over a range of angular orientations of the measurement object;2) reduce changes in direction of the exit measurement beam caused bychanges in the angular orientation of the measurement object; 3) reducea transverse displacement between the exit reference and measurementbeams caused by changes in angular orientation of the measurementobject; 4) reduce a transverse displacement between the exit referenceand measurement beams caused by translations of the measurement object;5) to insure that the measurement beam contacts the measurement objectat substantially normal incidence over a range of angular orientationsof the measurement object; 6) to reduce a transverse displacement of themeasurement beam caused by translations of the measurement object; and7) to reduce a transverse displacement of the measurement beam caused bychanges in angular orientation of the measurement object.

In general, in another aspect, the invention features an interferometrysystem including an interferometer which during operation directs areference beam along a reference path and a measurement beam along ameasurement path contacting a measurement object, and combines thereference and measurement beams to form overlapping exit reference andmeasurement beams. The overlapping exit measurement and reference beamsare indicative of changes in a relative optical path length between thereference and measurement paths. The interferometer includes a beamsteering assembly positioned to direct at least one of the reference andmeasurement beams. The system further includes a control circuit whichduring operation causes the beam steering assembly to redirect the atleast one of the reference and measurement beams in response to changesin angular orientation of the measurement object based on a signalgenerated from the exit measurement beam.

In general, in another aspect, the invention features a single-passinterferometry system including an interferometer which during operationdirects a reference beam along a reference path and a measurement beamalong a measurement path contacting a measurement object, and combinesthe reference and measurement beams to form overlapping exit referenceand measurement beams. The overlapping exit measurement and referencebeams are indicative of changes in a relative optical path lengthbetween the reference and measurement paths. The measurement pathcontacts the measurement object only a single time. The interferometerincludes a beam steering assembly positioned direct at least one of thereference and measurement beams. The system further includes a controlcircuit which during operation causes the beam steering assembly toredirect the at least one of the reference and measurement beams inresponse to changes in angular orientation of the measurement object.

In another aspect, the invention also features a lithography system foruse in fabricating integrated circuits on a wafer. The lithographysystem includes: a stage for supporting the wafer; an illuminationsystem for imaging spatially patterned radiation onto the wafer; apositioning system for adjusting the position of the stage relative tothe imaged radiation; and at least one of any of the interferometrysystems described above. The interferometry system(s) measures theposition of the wafer relative to the imaged radiation.

In another aspect, the invention features a lithography system for usein fabricating integrated circuits on a wafer. The lithography systemincludes a stage for supporting the wafer and an illumination system.The illumination system includes a radiation source, a mask, apositioning system, a lens assembly, and at least one of any of theinterferometry systems described above. During operation the sourcedirects radiation through the mask to produce spatially patternedradiation. The positioning system adjusts the position of the maskrelative to the radiation from the source. The lens assembly images thespatially patterned radiation onto the wafer. The interferometrysystem(s) measures the position of the mask relative to the radiationfrom the source.

In another aspect, the invention features a lithography system forfabricating integrated circuits. The lithography system includes firstand second components, the first and second components being movablerelative to each other. The lithography system also includes at leastone of any of the interferometry systems described above, wherein thefirst component includes the measurement object and the interferometrysystem(s) monitors the position of the first component relative to thesecond component.

In another aspect, the invention features a beam writing system for usein fabricating a lithography mask. The beam writing system includes: asource providing a write beam to pattern a substrate; a stage supportingthe substrate; a beam directing assembly for delivering the write beamto the substrate; a positioning system for positioning the stage andbeam directing assembly relative one another; and at least one of any ofthe inteferometry systems described above for measuring the position ofthe stage relative to the beam directing assembly.

In general, in another aspect, the invention features a method forinterferometry. The interferometry method includes: directing areference beam along a reference path and a measurement beam along ameasurement path contacting a measurement object; combining thereference and measurement beams to form overlapping exit reference andmeasurement beams, the overlapping exit measurement and reference beamsindicative of changes in a relative optical path length between thereference and measurement paths; and using an electronic control systemto redirect the measurement beam in response to changes in at least oneof angular orientation and position of the measurement object.

In general, in another aspect, the invention features a method forinterferometry. The interferometry method includes: receiving one ormore input beams to define a reference beam and a measurement beam;directing the reference beam along a reference path and the measurementbeam along a measurement path contacting a measurement object; combiningthe reference and measurement beams to form an output beam includingoverlapping exit reference and measurement beams, the overlapping exitmeasurement and reference beams indicative of changes in a relativeoptical path length between the reference and measurement paths; andusing an electronic control system to redirect the input and outputbeams in response to changes in at least one of angular orientation andposition of the measurement object.

In another aspect, the invention features a lithography method for usein fabricating integrated circuits. The lithography method includes:imaging spatially patterned radiation onto a wafer; positioning thewafer relative to the imaged radiation; and measuring the position ofthe wafer relative to the imaged radiation using at least one of theinterferometry methods described above.

In another aspect, the invention features a lithography method for usein the fabrication of integrated circuits. The lithography methodincludes: directing input radiation through a mask to produce spatiallypatterned radiation; positioning the mask relative to the inputradiation; measuring the position of the mask relative to the inputradiation using at least one of the interferometry methods describedabove; and imaging the spatially patterned radiation onto a wafer.

In another aspect, the invention features a lithography method for usein fabricating integrated chips. The lithography method includes:positioning a first component of a lithography system relative to asecond component of a lithography system to expose a wafer to spatiallypatterned radiation; and measuring the position of the first componentrelative to the second component using any of the interferometry methodsdescribed above.

In another aspect, the invention features a beam writing method for usein fabricating a lithography mask. The method includes: directing awrite beam to a substrate to pattern the substrate; positioning thesubstrate relative to the write beam; and measuring the position of thesubstrate relative to the write beam using any of the interferometrymethods described above.

Finally, in general, in another aspect, the invention features aninterferometry system for measuring changes in distance to a measurementobject. The interferometry system includes an interferometer, a beamsteering assembly for redirecting at least one beam within theinterferometer, and a control circuit for reorienting the beam steeringassembly in response to changes in the angular orientation or positionof the measurement object.

Embodiments of the interferometry systems and methods described aboveinclude many advantages.

For example, the system can maintain the exit reference and measurementbeams substantially parallel to one another over a range of orientationsof the measurement object, and can do so with only a single pass of themeasurement beam to the measurement object. The single-pass systemreduces the bandwidth of electronics needed to process electricalinterference signals having Doppler shifts, relative to those of adouble-pass interferometer. Furthermore, the single-pass system reducesthe likelihood of depolarization, scattering, and undesired spuriousreflections from transmissive optics within the interferometer, relativeto those for a double-pass interferometer. Such effects can introduceerrors, e.g., cyclic errors, in the measured phase of the electricalinterference signal.

Also, the system can minimize the transverse displacement of the exitreference and measurement beams or the transverse displacement ofcomponents of the reference and measurement beams within theinterferometer caused by changes in the angular orientation or positionof the measurement object. As a result, the average amplitude of theelectrical interference signal produced from the mixed exit referenceand measurement beams can be substantially independent of changes in theangular orientation and position of the measurement object. In addition,the system reduces changes of the paths of the measurement and referencebeams through transmissive optics of the interferometer, e.g.,polarizing beam splitters and quarter wave plates, caused by changes inthe angular orientation or position of the measurement object. Suchtransmissive optics can have imperfections in their surface figures andlocal variations in refractive index. Thus, changes in the paths of thebeams through such optics can change the optical path length measured bythe interferometer even though the distance between the measurementobject and the interferometer has not changed. Such negative effects canbe compounded if the transmissive optic has dispersive properties, suchas those produced by a wedge.

Furthermore, in some embodiments, the system includes one or morepolarizing beam splitters as the only transmissive optics, with allother optics being reflective. In particular, quarter wave plates arenot required. Minimizing the number of transmissive optics in the systemoptics reduces depolarization, scattering, and undesired spuriousreflections of the reference and measurement beams, which can introduceerrors, e.g., cyclic errors, in the measured phase of the electricalinterference signal.

Other features, aspects, and advantages will be clear from the followingdetailed description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 f are schematic drawings of a first embodiment of theinterferometry system. FIGS. 1a and 1 b depict beam propagation in lowerand upper planes, respectively. FIG. 1c depicts beam propagation fromthe left end of the system shown in FIGS. 1a and 1 b, the plane of FIG.1c being orthogonal to the planes of FIGS. 1a and 1 b. FIG. 1d is across-sectional view of beams entering and exiting a component of thesystem. FIG. 1e is a schematic drawing of a detector system and FIG. 1fis a diagram used to discuss path length variations.

FIGS. 2a-2 f are schematic drawings of a second embodiment of theinterferometry system. FIGS. 2a and 2 d depict beam propagation in upperand lower planes, respectively. FIG. 2c depicts beam propagation as seenfrom the left end of the system shown in FIGS. 2a and 2 d, the plane ofFIG. 2c being orthogonal to the planes of FIGS. 2a and 2 d. FIG. 2b isan inset for FIG. 2a. FIG. 2e depicts beam propagation as seen from abottom view of portions of system shown in FIGS. 2a and 2 d. FIG. 2f isa cross-sectional view of beams entering and exiting a component of thesystem.

FIGS. 3a-3 f are schematic drawings of a third embodiment of theinterferometry system. FIGS. 3a and 3 e are insets for FIG. 3d. FIGS. 3band 3 d depict beam propagation in upper and lower planes, respectively.FIG. 3c depicts beam propagation orthogonal to the planes of FIGS. 3band 3 d. FIG. 3f is a cross-sectional view of the beams as seen from theright end of the system shown in FIGS. 3b and 3 d in a plane orthogonalto the planes of FIGS. 3b and 3 d and also orthogonal to the plane ofFIG. 3c.

FIGS. 4a-4 f are schematic drawings of a fourth embodiment of theinterferometry system. FIGS. 4a and 4 e are insets for FIG. 4d. FIGS. 4band 4 d depict beam propagation in upper and lower planes, respectively.FIG. 4c depicts beam propagation orthogonal to the planes of FIGS. 4band 4 d. FIG. 4f is a cross-sectional view of beams as seen from theright side of FIG. 4b in a plane orthogonal to the planes of FIGS. 4band 4 d and also orthogonal to the plane of FIG. 4c.

FIG. 5 is a schematic drawing of a fifth embodiment of theinterferometry system.

FIGS. 6a-6 b are schematic drawings of a sixth embodiment of theinterferometry system. FIGS. 6a and 6 b depict beam propagation inorthogonal planes.

FIGS. 7a-7 b are schematic drawings of a seventh embodiment of theinterferometry system. FIGS. 7a and 7 b depict beam propagation inorthogonal planes.

FIGS. 8a-8 c are schematic drawings of an eighth embodiment of theinterferometry system. FIGS. 8b and 8 c depict beam propagation inplanes that are orthogonal to the plane of FIG. 8a.

FIGS. 9a-9 c are schematic drawings of a ninth embodiment of theinterferometry system. FIGS. 9b and 9 c depict beam propagation inplanes that are orthogonal to the plane of FIG. 9a.

FIGS. 10a-10 c are schematic drawings of a tenth embodiment of theinterferometry system. FIGS. 10b and 10 c depict beam propagation inplanes that are orthogonal to the plane of FIG. 10a.

FIGS. 11a-11 f are schematic drawings of an eleventh embodiment of theinterferometry system. FIGS. 11b and 11 c depict beam propagation inplanes that are orthogonal to the plane of FIG. 11a. FIG. 11d is anextension of FIG. 11a. FIGS. 11e and 11 f are insets of FIG. 11d.

FIGS. 12a-12 e are schematic drawings of a twelfth embodiment of theinterferometry system. FIGS. 12b depicts beam propagation in a planethat is orthogonal to the plane of FIG. 12a. FIG. 12c is an extension ofFIG. 12a. FIGS. 12d and 12 e are insets of FIG. 12c.

FIGS. 13a-13 b are schematic drawings of a thirteenth embodiment of theinterferometry system. FIG. 13b depicts beam propagation in a plane thatis orthogonal to the plane of FIG. 13a.

FIG. 14a-c relate to lithography and its application to manufacturingintegrated circuits. FIG. 14a is a schematic drawing of a lithographyexposure system employing the interferometry system. FIGS. 14b and 14 care flow charts describing steps in manufacturing integrated circuits.

FIGS. 15a-15 d are schematic drawings of additional embodiments of theinterferometry system.

FIG. 16 is a schematic of a beam writing system employing theinterferometry system.

DETAILED DESCRIPTION

The invention features an interferometry system including aninterferometer and at least one beam steering assembly controlled by acontrol circuit. In some embodiments, the beam steering assembly is acomponent of the interferometer, directing at least one of the referenceand measurement beams within the interferometer. In other embodiments,the beam steering assembly is separate from the interferometer,directing an input beam to, and an output beam from, the interferometer.The beam steering assembly redirects one or more of the input, output,reference, and measurement beams to minimize undesired consequences ofchanges in the angular orientation or position of the measurementobject.

Four embodiments of the invention will be first described wherein theseembodiments involve relatively few components and beam propagation inonly one plane. Nonetheless, these embodiments demonstrate importantfeatures of the invention that will be expanded upon in additionalembodiments described subsequently.

A schematic of an embodiment of the invention is shown in FIG. 15a. Apolarizing beam splitter 1512 in an interferometer 1510 receives aninput beam 1514 from a laser source (not shown) and separates input beam1514 into reference beam 1516 (dotted line) and measurement beam 1518(solid line), which are linearly polarized orthogonal to one another.The polarizing beam splitter 1512 includes a polarizing interface 1562for reflecting beams having a polarization orthogonal to the plane ofFIG. 15a and a back reflective surface 1564 for reflecting beamstransmitted through polarizing interface 1562.

In the described embodiment, the orthogonally polarized. reference andmeasurement beams are frequency-shifted with respect one another, e.g.,by Zeeman splitting, acousto-optic modulation, or by use of birefringentelements internal to the laser, prior to entering interferometer 1510 ascomponents of input beam 1514. The different frequencies enable theinterferometry system to generate a heterodyne interference signal.Although the described embodiment is a heterodyne system, the instantinvention is readily adapted for use in a homodyne system in which thereference and measurement beams have the same frequencies.

Polarizing beam splitter 1512 directs the measurement beam 1518 to abeam steering assembly 1520, which includes a beam steering mirror 1522and a pair of piezoelectric transducers 1524 and 1526. The transducersare coupled to the beam steering mirror 1522 by flexures to orient thebeam steering mirror in response to a signal 1528 from a servocontroller 1530. The beam steering assembly may include capacitancegauges to measure changes in orientation and/or position of the beamsteering mirror 1522. The capacitance gauges may also be used to measureand/or monitor the properties of the piezoelectric transducers 1524 and1526.

Beam steering assembly 1520 directs the measurement beam throughreference retroreflector 1532, reference retroreflector 1532 beingtruncated so that a beam passing centrally through retroreflector 1532is not retroreflected, to contact a stage mirror 1534, i.e., themeasurement object, at substantially normal incidence. Stage mirror 1534then reflects the measurement beam back to retrace its path to beamsteering assembly 1520 and polarizing beam splitter 1512. Themeasurement beam double passes a quarter wave plate 1536, which ispositioned between the beam steering assembly 1520 and polarizing beamsplitter 1512 and which rotates the linear polarization of themeasurement beam by 90°.

Polarizing beam splitter 1512 directs the reference beam 1516 to thebeam steering assembly 1520, which in turn directs the reference beam toreference retroreflector 1532. The reference retroreflector then directsthe reference begin back to beam steering assembly 1520 and ontopolarizing beam splitter 1512. The reference beam also double passesquarter wave plate 1536, which rotates the linear polarization of thereference beam by 90°.

Polarizing beam splitter 1512 then recombines the polarization-rotatedreference and measurement beams to form overlapping exit reference andmeasurement beams, which together form an output beam 1540. A beamsplitter 1542 sends a portion of output beam 1540 to a detector system1544, which measures the difference in directions of propagation of theexit reference and measurement beams. The detector system sends an errorsignal 1550 indicative of any such difference to servo controller 1530,which sends signal 1528 to beam steering assembly 1520 in response tothe error signal. The beam steering assembly 1520 changes, in responseto signal 1528, the orientation of beam steering mirror 1522 preferablyabout a nodal point of reference retroreflector 1532, changes inorientation of beam steering mirror 1522 about a nodal point ofreference retroreflector 1532 producing substantially reduced lateralshear effects experienced by the reference beam.

Alternatively when the direction of the input beam 1514 is constant,detector system 1544 can measure the difference in position of the exitmeasurement beam from a reference position in the detector system andgenerate error signal 1550 indicative of that difference in position, adifference in position of the exit measurement beam being theconsequence of a change in direction of propagation of the exitmeasurement beam component of output beam 1540. For example, thereference position can be the position of the exit measurement beam inthe detector system corresponding to the measurement beamretroreflecting from stage mirror 1534, i.e., contacting the stagemirror at normal incidence, and the stage mirror 1534 being at a nominalnull orientation. In other embodiments, the detector system can includemultiple detectors for determining the direction and position of theexit reference and measurement beams and can generate an error signalbased on such information.

Changes in the angular orientation of stage mirror 1534 change thedirection of the measurement beam and the direction of subsequent exitmeasurement beam. This causes detector system 1544 to generate errorsignal 1550. Servo-controller 1530 responds to the error signal bydirecting beam steering assembly 1520 to reorient beam steering mirror1522 so as to minimize the error signal, e.g., by directing themeasurement beam to the stage mirror at normal incidence. As a result,the exit reference and measurement beams remain substantially parallelto one another and the position of the exit measurement beam remainssubstantially constant over a range of angular orientations of the stagemirror. Furthermore, since beam steering assembly 1520 redirects each ofthe reference and measurement beams twice, there is no change in therelative optical path length between the reference and measurement beamsto first order in a translation of beam steering mirror 1522 in adirection normal to the reflecting surface of beam steering mirror 1522.In addition, when the measurement beam is directed to the stage mirrorat normal incidence and the paths of the measurement and reference beamshave substantially the same centroids, there is no change in therelative optical path length between the reference and measurement beamsto first order in any angular orientation change of the stage mirror(about the center of a measurement beam spot on the stage mirror) andany corresponding change in the beam steering element.

The remainder of output beam 1540, after beam splitter 1542 passesthrough a polarizer 1545, which mixes polarizations of the exitreference and measurement beams to form a mixed beam 1546. A signalprocessing system 1548 measures the intensity of the mixed beam, e.g.,by photoelectric detection, to produce an electrical interference signalor electrical heterodyne signal, extracts the phase of the electricalheterodyne signal, and determines from that phase the difference inoptical path length between the paths of the reference and measurementbeams.

In other embodiments, detector system 1544 could be part of a separateangle sensing system (not shown), comprised of an additional light beam(not shown) directed towards stage mirror 1534. The separate anglesensing system would measure the absolute or relative angularorientation of stage mirror 1534 using the additional light beam andwould send a measurement signal to servo-controller 1530 indicative ofthat orientation. As in the embodiment of FIG. 15a, servo-controller1530 would control beam steering assembly 1520 based on the signal fromthe detector system. In addition, in embodiments in which severalinterferometry systems each direct a measurement beam to a measurementobject, a single angle sensing system can be used to guide control ofthe beam steering assembly in each of the interferometry systems.

In another embodiment similar to that of FIG. 15a, a polarizing beamsplitter 1512 and the beam steering assembly 1520 are combined into asingle unit, as shown by polarizing beam splitter and beam steeringassembly 1560 in FIG. 15b. In this variation, transducers 1524 and 1526are coupled through flexures directly to polarizing beam splitter 1512.As a result, servo controller 1530 orients the polarizing interface 1562and reflective back surface 1564 of polarizing beam splitter 1512 viatransducers 1524 and 1526 to direct the reference and measurement beamsto reference retroreflector 1532 and stage mirror 1534, respectively.Quarter wave plate 1536 is positioned between reference retroreflector1532 and beam steering assembly 1560. Other features of this embodimentare the same as those of the embodiment in FIG. 15a and are numbered thesame.

In a further embodiment, similar to that of FIG. 15a and shown in FIG.15c, input beam 1514 first contacts beam steering assembly 1520, whichthen directs the input beam into the interferometer 1570. In thisembodiment, the interferometer includes polarizing beam splitter 1512,quarter wave plate 1536, and reference retroreflector 1532, but does notinclude beam steering assembly 1520, which is outside theinterferometer. As in the embodiment of FIG. 15b, the polarizing beamsplitter 1512 directs the reference and measurement beams to referenceretroreflector 1532 and stage mirror 1534, respectively, and recombinesthe reflected reference and measurement beams to form the overlappingexit reference and measurement beams, which are the components of outputbeam 1540. Beam steering assembly 1520 then receives output beam 1540and directs it to detection system 1544 and signal processing system1548, as in the embodiment of FIG. 15a.

As in the embodiment of FIG. 15a, servo controller 1530 in FIG. 15corients beam steering mirror 1522 of assembly 1520 to minimize the errorsignal generated by detection system 1544. To do so, servo controller1530 orients beam steering mirror 1522 of assembly 1520 to direct theinput beam into interferometer 1570 such that the measurement bearcontacts stage mirror 1534 at normal incidence over a range of angularorientations of the stage mirror. The beam steering assembly 1520changes, in response to signal 1528, the orientation of beam steeringmirror 1522 preferably about a nodal point of reference retroreflector1532, changes in orientation of beam steering mirror 1522 about a nodalpoint of reference retroreflector 1532 producing substantially reducedlateral shear effects (tranverse displacement) on the reference beam. Asa result, the exit reference and measurement beams, which form theoutput beam, emerge from the interferometer substantially parallel.Changes in the direction of the output beam emerging from theinterferometer caused by the beam steering assembly redirecting theinput beam are compensated when the output beam contacts the beamsteering assembly, which directs the output beam to detection system1544 and signal processing system 1548.

In another embodiment similar to FIG. 15a and shown in FIG. 15d, theinterferometry system receives a pair of spatially separated input beamsto define the reference and measurement beams rather than separating asingle input beam into the reference and measurement beams. In thisembodiment, an interferometer 1580 receives spatially separated inputbeams 1582 and 1584 polarized orthogonal to one another and a polarizingbeam splitter 1586 directs the received input beams along differentpaths to define reference beam 1516 and measurement beam 1518. Prior toentering polarizing beam splitter 1586, a portion of the input beams issplit off by a non-polarizing beam splitter 1588 and directed to areference phase detector 1590. The reference phase detector includesoptics to optically mix the input beams and electronics to measure therelative optical phase of the mixed beams. Reference phase detector 1590sends a reference phase signal 1592 indicative of the relative phasedifference between input beams 1582 and 1584 upon enteringinterferometer 1580 to signal processing system 1548, which uses signal1592 to determine the difference in optical path length between thepaths of the reference and measurement beams.

The remaining description for the embodiment of FIG. 15d is identical tothat for FIG. 15a with polarizing beam splitter 1586 recombiningreference beam 1516 and measurement beam 1518 to form output beam 1540.The embodiment of FIG. 15d and other such embodiments in which theinterferometer receives spatially separated input beams to define thereference and measurement beams reduces paths within optics of theinterferometer in which the reference and measurement beams overlap oneanother. Such a reduction reduces polarization mixing between thereference and measurement beams that can cause cyclic errors in theheterodyne signal measured by system 1548. For a reference describingsuch cyclic errors, see, e.g., N. Bobroff, Appl. Opt., 26:2676-2681,1986. Also, in other embodiments such as that of FIG. 15c in which thebeam steering assembly is outside the interferometer, the beam steeringmirror can direct spatially separated input beams into theinterferometer to define the reference and measurement beams.

The concepts inherent in the systems described above and shown in FIGS.15a-15 d can be extended to other applications. For example, otherembodiments can address changes in the angular orientation and positionof the measurement object along multiple dimensions. Also, otherembodiments can insure that the transverse spatial profile of the exitreference beam is not inverted relative to that of the exit measurementbeam when these beams are mixed to produce the interference signal.Also, other embodiments can include additional beam steering assemblies.Such alternative embodiments are described further below.

In general, these embodiments relate to apparatus and methods formeasuring and monitoring a linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object,wherein the lateral displacement is orthogonal to the lineardisplacement. An example of an important application is in using aninterferometry system to measure and monitor along a given dimension thelocation of one or more stages in a lithographic tool that is steppingand/or scanning in either one, two, or three orthogonal dimensions.

The embodiments described below comprise an interferometer and at leastone beam steering assembly, which can be a component of, or separatefrom, the interferometer. The embodiments generally fall into one ofthree classes. In the first class, the beam steering assembly ispositioned to direct components of both a measurement beam and areference beam. In the second class, the beam steering assembly ispositioned to direct components of the measurement but not the referencebeam. And, in the third class, the beam steering assembly is positionedto direct an input beam to and an output beam from, the interferometer.

Within each class, the interferometry systems can function in one orboth of two different modes of operation. The first mode of operationcompensates for changes in the direction of the measurement beam or exitmeasurement beam caused by changes in the angular orientation of themeasurement object. The changes in angular orientation can be along oneor more directions, e.g., changes in pitch and yaw. The second mode ofoperation compensates for changes in the transverse position of themeasurement beam or exit measurement beam caused by changes in theposition of the measurement object. The changes in position can be alongany of two transverse directions, i.e., directions orthogonal to thedisplacement being measured, and a longitudinal direction, i.e., adirection along the displacement being measured.

The modes of operation for a particular interferometry system depend onthe reflective properties of the measurement object. For example, if themeasurement beam reflects from a plane mirror, changes in the angularorientation of the plane mirror about two orthogonal axes in the planeof the mirror causes changes the direction of the reflected measurementbeam relative to the two axes. However, translations of the plane mirroralong either of these two axes do not affect the direction or transverseposition of the reflected measurement beam. Therefore, for applicationsin which the measurement beam reflects from a plane mirror of themeasurement object, the system typically operates in the first mode.

In a different example, if the measurement beam reflects from aretroreflector, the direction of the reflected measurement beam isindependent of changes in the angular orientation or position of themeasurement object. However, translations of the retroreflector alongdirections orthogonal to an incoming measurement beam cause changes inthe transverse position of the reflected measurement beam. Therefore,for an application in which the measurement beam retroreflects from ameasurement object in a given plane, the system typically operates inthe second mode for translations of the measurement object in that planeand orthogonal to the incoming measurement beam.

As would be clear to those skilled in the art, applications in which themeasurement object includes a roof prism to reflect the measurement beamcombine features of the previous two examples. In such an application,the system operates in both the first and second modes.

The system can also operate in both first and second modes forapplications in which the measurement object includes a plane mirror toreflect the measurement beam, and the measurement beam contacts theplane mirror at non-normal incidence. In such applications longitudinaltranslations of the measurement object change the transverse position ofthe reflected measurement beam and changes in the angular orientation ofthe measurement object change the direction of the reflected measurementbeam.

Embodiments of the interferometry system can also include single andmultiple passes of the measurement beam to the measurement object. Insingle-pass systems, changes in the angular orientation or position ofthe measurement object cause changes in the direction or transverseposition of the exit measurement beam. The changes to the exitmeasurement beam as detected by a detector system provide the basis foran error signal used to control the beam steering assembly. In somemultiple pass arrangements, changes in the angular orientation orposition of the measurement object do not change the direction and/ortransverse position of the exit measurement beam but do change thedirection and/or transverse position of the measurement beam within theinterferometer. To compensate for changes in the direction and/ortransverse position of the measurement beam within the interferometer,some embodiments of the interferometry system split off an intermediatebeam from the measurement beam after the measurement beam has contactedthe measurement object an odd number of times. The direction and/orposition of the intermediate beam as detected by a detector systemprovide the basis for an error signal used to control the beam steeringassembly.

For each of the embodiments of the present invention, the interferometerand at least one beam steering assembly are configured such that adisplacement of certain optical elements of the at least one beamsteering assembly as a unit does not effect in at least first order thedifference between the optical path length of the measurement andreference beams. For the first mode of operation, the interferometer andat least one beam steering assembly are configured such that changes inthe orientations of the measurement object and/or of certain opticalelements of a beam steering assembly as a unit substantially do noteffect in at least first order the difference between the optical pathlength of the measurement and reference beams. For the second mode ofoperation, the interferometer and at least one beam steering assemblyare configured such that a displacement in the location of themeasurement object orthogonal to the linear displacement substantiallydoes not effect in at least first order the difference between theoptical path length of the measurement and reference beams.

FIGS. 1a-1 f depict, in schematic form, a first embodiment of thepresent invention for measuring a linear displacement of a measurementobject substantially independent of the orientation of the measurementobject and substantially independent of a lateral displacement of themeasurement object, the lateral displacement being orthogonal to thelinear displacement. The first embodiment is from the first class ofembodiments and variants thereof of the present invention, which operatein the first mode of operation for changes in orientation of themeasurement object about two orthogonal axes.

The first embodiment comprises light beam 110, a beam steering assembly,a first and second beam splitting assemblies, retroreflector 172,measurement object mirror 170 comprised of a single reflecting surface,and reference beam mirror 178 comprised of a single reflecting surface.Prism 150 and rhomboidal prism 1524 comprise the first beam splittingassembly depicted in FIG. 1a wherein interface 152 is a polarizing beamsplitting interface and surface 156 is a reflector. Rhomboidal prism160, prisms 166 and 186, and dove prism 182 comprise the second beamsplitting assembly depicted in FIG. 1b wherein interface 162 is areflector, interfaces surfaces 164 and 174 are polarizing beam splittinginterfaces, interface surface 184 in a nonpolarizing beam splittinginterface, and interface surface 180 shown as a dotted line in FIG. 1bis antireflection coated.

The first embodiment is adapted for use as a heterodyne interferometersystem. A source (not shown in FIGS. 1a-1 f) provides for the firstembodiment an entrance beam 110 comprised of two orthogonally polarizedcomponents of differing frequencies.

The light source such as a laser can be any of a variety of frequencymodulation apparatus and/or lasers. For example, the laser can be a gaslaser, e.g. a HeNe, stabilized in any of a variety of conventionaltechniques known to those skilled in the art, see for example, T. Baeret al., “Frequency Stabilization of a 0.633 μm He-Ne-longitudinal ZeemanLaser,” Applied Optics, 19, 3173-3177 (1980); Burgwald et al., U.S. Pat.No. 3,889,207, issued Jun. 10, 1975; and Sandstrom et al., U.S. Pat. No.3,662,279, issued May 9, 1972. Alternatively, the laser can be a diodelaser frequency stabilized in one of a variety of conventionaltechniques known to those skilled in the art, see for example, T. Okoshiand K. Kikuchi, “Frequency Stabilization of Semiconductor Lasers forHeterodyne-type Optical Communication Systems,” Electronic Letters, 16,179-181 (1980) and S. Yamaqguchi and M. Suzuki, “SimultaneousStabilization of the Frequency and Power of an AlGaAs SemiconductorLaser by Use of the Optogalvanic Effect of Krypton,” IEEE J. QuantumElectronics, QE-19, 1514-1519 (1983).

Two optical frequencies may be produced by one of the followingtechniques: (1) use of a Zeeman split laser, see for example, Bagley etal., U.S. Pat. No. 3,458,259, issued Jul. 29, 1969; G. Bouwhuis,“Interferometric Mit Gaslasers,” Ned. T. Natuurk, 34, 225-232 (August1968); Bagley et al., U.S. Pat. No. 3,656,853, issued Apr. 18, 1972; andH. Matsumoto, “Recent interferometric measurements using stabilizedlasers,” Precision Engineering, 6(2), 87-94 (1984); (2) use of a pair ofacousto-optical Bragg cells, see for example, Y. Ohtsuka and K. Itoh,“Two-frequency Laser Interferometer for Small Displacement Measurementsin a Low Frequency Range,” Applied Optics, 18(2), 219-224 (1979); N.Massie et al., “Measuring Laser Flow Fields With a 64-Channel HeterodyneInterferometer,” Applied Optics, 22(14), 2141-2151 (1983); Y. Ohtsukaand M. Tsubokawa, “Dynamic Two-frequency Interferometry for SmallDisplacement Measurements,” Optics and Laser Technology, 16, 25-29(1984); H. Matsumoto, op. cit.; P. Dirksen, et al., U.S. Pat. No.5,485,272, issued Jan. 16, 1996; N. A. Riza and M. M. K. Howlader,“Acousto-optic system for the generation and control of tunablelow-frequency signals,” Opt. Eng., 35(4), 920-925 (1996); (3) use of asingle acousto-optic Bragg cell, see for example, G. E. Sommargren,commonly owned U.S. Pat. No. 4,684,828, issued Aug. 4, 1987; G. E.Sommargren, commonly owned U.S. Pat. No. 4,687,958, issued Aug. 18,1987; P. Dirksen, et al., op. cit.; (4) use of two longitudinal modes ofa randomly polarized HeNe laser, see for example, J. B. Ferguson and R.H. Morris, “Single Mode Collapse in 6328 Å HeNe Lasers,” Applied Optics,17(18), 2924-2929 (1978); or (5) use of birefringent elements or thelike internal to the laser, see for example, V. Evtuhov and A. E.Siegman, “A “Twisted-Mode” Technique for Obtaining Axially UniformEnergy Density in a Laser Cavity,” Applied Optics, 4(1), 142-143 (1965).

The specific device used for the source of beam 110 will determine thediameter and divergence of beam 110. For some sources, e.g. a diodelaser, it will likely be necessary to use conventional beam shapingoptics, e.g. a conventional microscope objective, to provide beam 110with a suitable diameter and divergence for the elements that follow.When the source is a HeNe laser, for example, beam shaping optics maynot be required.

Beam 110 impinges on the first beam splitting assembly, as illustratedin FIG. 1a, with a portion of beam 110 transmitted by interface 152 andreflected by surface 156 as measurement beam 111. Beam 111 is polarizedin the plane of FIG. 1a. A second portion of beam 110 is reflected byinterface 152 as reference beam 112 orthogonally polarized to the planeof FIG. 1a.

Beam 111 enters beam steering element 158 (cf. FIG. 1a) and exits beamsteering element 158 as beam 113 (cf. FIG. 1b) displaced laterally frombeam 111 and propagating in a direction nominally opposite to thedirection of propagation of beam 111. The projections of beam 113 andbeam 111 in the plane of FIG. 1b are superimposed. In beam steeringelement 158, beam 111 is reflected in sequential order, e.g., by totalinternal reflection, by each of the surfaces 158A, 158B, and 158C (cf.FIG. 1c which presents an end view of the interferometer depicted inFIG. 1a and which shows the path of reference beam 112 in beam steeringelement 158). A beam steering assembly that redirects measurement beam113 comprises beam steering element 158 and orientation/translationtransducer 159. Orientation/translation transducer 159 preferably altersthe orientation of beam steering element 158 about a nodal point ofretroreflector 172 for a change in orientation in the plane of FIG. 1c,and/or alters the location of steering element 158 in the nominaldirection of propagation of beam 111, wherein changes in orientation ofbeam steering element 158 about a nodal point of retroreflector 172substantially reduce lateral shear effects experienced by the referencebeam. The orientation of beam steering element 158 is controlled byelectronic signal 140 and the location of beam steering element 158 iscontrolled by electronic processor and computer 194 through electronicsignal 144.

Beam 113 enters the second beam splitting assembly, depicted in FIG. 1b,wherein it is reflected at interface 162 as beam 115 and thentransmitted by polarizing beam splitting interface 164 as beam 117. Beam117, polarized in the plane of FIG. 1b, is transmitted by quarter wavephase retardation plate 168 as a circularly polarized beam, reflected bymeasurement object mirror 170 as a beam with opposite-handed circularpolarization, and subsequently transmitted by phase retardation plate168 as beam 119 linearly polarized orthogonal to the plane of FIG. 1b.Beam 119 enters the second beam splitting assembly and is reflected bypolarizing beam splitting interface 164 as beam 121.

Beam 121 exits the second beam splitting assembly, enters beam steeringelement 158 (cf. FIG. 1b), and exits beam steering element 158 as beam123 (cf. FIG. 1a) propagating in a direction nominally opposite to thedirection of propagation of beam 121. Beam 123 enters retroreflector 172(cf. FIG. 1a) and is retroreflected as beam 125 (cf. FIG. 1b). Beams 123and 125 are shown in FIG. 1d as entering and exiting beams,respectively, to and from retroreflector 172, respectively.

Reference beam 112 enters beam steering element 158 (cf. FIG. 1a) andexits beam steering element 158 as beam 114 (cf. FIGS. 1b and 1 c)propagating in a direction nominally opposite to the direction ofpropagation of beam 112. The projections of beams 114 and beam 112 aresuperimposed in the plane of FIG. 1a. The projections of the paths ofbeams 111 and 112 through beam steering element 158 are superimposed inthe plane of FIG. 1c. Beam 114 enters retroreflector 172 (cf. FIG. 1b)and is retroreflected as beam 116 (cf. FIG. 1a). In FIG. 1d, beams 114and 116 are shown as entering and exiting beams, respectively, to andfrom retroreflector 172, respectively. Beam 116 enters beam steeringelement 158 (cf. FIG. 1a) and exits beam steering element 158 as beam118 (cf. FIG. 1b) propagating in a direction nominally opposite to thedirection of propagation of beam 116.

Beam 118 enters the second beam splitting assembly and a portion of beam118 is reflected by polarizing beam splitting interface 174 as beam 120(cf. FIG. 1b). Beam 120, orthogonally polarized to the plane of FIG. 1b,is transmitted by quarter wave phase retardation plate 176 as acircularly polarized beam, reflected by reference mirror 178 as a beamwith opposite-handed circular polarization, and subsequently transmittedby quarter wave phase retardation plate 176 as beam 122 linearlypolarized in the plane of FIG. 1b. Beam 122 enters the second beamsplitting assembly and 122 is transmitted by polarizing beam splittinginterface 174 as beam 124. Beam 124 is transmitted by antireflectioncoated interface 180 as beam 126.

Measurement beam 125 enters, as shown in FIG. 1b, the second beamsplitting assembly and a first portion of beam 125 is transmitted bynonpolarizing beam splitting interface 184 as one component of a firstoutput beam 128. A first portion of beam 126 is reflected bynonpolarizing beam splitter interface 184 as a second component of thefirst output beam 128. First output beam 128 is reflected by reflectinginterface 162 as output beam 130. Output beam 130 impinges on detector188 to produce an electrical signal 140 with two components related toany difference in the directions of propagation of measurement andreference beam components in output beam 130 in two orthogonal planes,respectively.

A second portion of beam 125 is reflected by nonpolarizing beamsplitting interface 184 as one component of a second output beam 132. Asecond portion of beam 126 is transmitted by nonpolarizing beamsplitting interface 184 as a second component of the second output beam132. Beam 132 is transmitted by a polarizer 190 orientated to produce amixed optical beam 134. Beam 134 impinges on detector 192 to generateelectrical signal 142 related to the difference in phase of themeasurement and reference beam components of mixed beam 134. Electricalsignal 142 is transmitted to electronic processor and computer 194 forsubsequent processing to obtain information about the difference inphase of the measurement and reference beam components of mixed beam134. The detection of mixed beam is typically by photoelectric detectionto produce a heterodyne signal, the phase of which is related to thedifference in optical path lengths of the measurement and referencebeams. The phase of the heterodyne signal may be determined byelectronic processor and computer 194 for example from a Fouriertransform or Hilbert transform of the heterodyne signal.

The elements of detector 188 are depicted, in schematic form, in FIG.1e. Beam 130 enters a polarizing beam splitter comprising prisms 188Aand 188C with polarizing interface 188B. The measurement beam componentof beam 130, being polarized orthogonal to the plane of FIG. 1e isreflected by polarizing interface 188B as beam 130M. Beam 130M isfocused by lens 188D to a spot on detector 188E such as a quad celldetector, a two-dimensional high speed CCD camera, or a lateral effectphotodiode. Two-dimensional data arrays are transmitted as electronicsignal 140A to electronic processor 188H for subsequent processing. Thelocation of the spot on detector 188E is not effected by a lateral shearof beam 130M but a change in the direction of propagation beam 130M doesshift the location of the spot on detector 188E. Thus the electronicsignal contains information about the direction of propagation of beam130M.

The reference beam component of beam 130, being polarized in the planeof FIG. 1e is transmitted by polarizing interface 188B as beam 130R.Beam 130R is focused by lens 188F to a second spot on a second detector188G such as a quad cell detector, a two-dimensional high speed CCDcamera, or a lateral effect photodiode. Two-dimensional data arrays aretransmitted as electronic signal 140B to electronic processor 188H forsubsequent processing. The location of the second spot on detector 188Gis not effected by a lateral shear of beam 130R but a change in thedirection of beam 130R does shift the location of the second spot ondetector 188G. Thus the electronic signal 130R contains informationabout the direction of propagation of beam 130R.

Electronic processor 188H processes the electronic signals 140A and 140Bto generate two electronic signals 140C and 140D where the twoelectronic signals 140C and 140D are proportional to the difference inthe directions of propagation of the measurement and reference beamcomponents of beam 130 in two orthogonal planes. Electronic signals 140Cand 140D comprise electronic signal 140.

Electrical signal 140 is transmitted to orientation/translationtransducer 159. Information contained in signal 140 is used as the errorsignal in a servo system to alter the orientation of beam steeringmirror 158 by orientation/translation transducer 159 so as to maintainin two dimensions the direction of propagation of measurement beam 123substantially constant independent of changes of orientation ofmeasurement object mirror 170 about two orthogonal axes. Under thecondition that the physical path length of the measurement beam from thelocation of the first redirection of the measurement beam by the beamsteering element 158 to measurement object mirror 170 is equal to thephysical path length of the measurement beam from measurement objectmirror 170 to the location of the second redirection of the measurementbeam by the beam steering mirror 158, there will be no lateral shear ofbeam 123 at beam steering element 158 when changes in orientation ofmeasurement object mirror 170 are compensated by changes in orientationof beam steering element 158. This condition with respect to physicalpath lengths will be referenced as the first embodiment condition.

Compliance with the first embodiment condition eliminates the lateralshear of measurement beam 123 at beam steering element 158 when changesin orientation of measurement object mirror 170 are compensated bychanges in orientation of beam steering element 158. However, there willbe introduced lateral shear of reference beam 126 when changes inorientation of measurement object mirror 170 are compensated by changesin orientation of beam steering element 158. Although a lateral shearwill be introduced in reference beam 126, the magnitude of the lateralshear is significantly reduced in relation to the lateral shearintroduced in measurement beam components of prior art plane mirrorinterferometers due to changes in orientation of the measurement objectmirror, the reduction factor being proportional to the ratio of thephysical length of the reference leg to the physical length of themeasurement leg of an interferometer.

For certain end use applications, it may be desirable to introduce atranslation of beam steering element 158 in a direction nominallyparallel to the direction of propagation of 123. A translation of thistype for beam steering element 158 is controlled by the signal processorand computer 194 through electrical signal 144 transmitted toorientation/translation transducer 159.

The inventive use of an active servo control of an adjustable elementmakes it possible to perform accurate displacement measurements tomeasurement object mirror 170 using a single pass of a measurement beamto measurement object mirror 170. For applications where the firstembodiment condition is satisfied to a certain relative precision, theinventive technique reduces (to the certain relative precision)measurement object mirror 170 orientation-dependent errors in the phaseof the measurement beam relative to the phase of reference beam causedby lateral beam shear and uncompensated dispersive properties of theelements of the interferometer.

The interferometer and beam steering assembly of the first embodimentare configured such that a displacement of beam steering element 158does not effect in at least in second order the optical path length ofthe measurement beam relative to the optical path length of thereference beam; both the reference and measurement beams have theirdirections of propagation redirected twice by beam steering element 158with substantially the same angles of incidence in each instance ofredirection, the centroids of the paths of the reference and measurementbeams entering beam steering element 158 are substantially coextensive,and the centroids of the paths of the reference and measurement beamsexiting beam steering element 158 are substantially coextensive. Anexample from the set of lowest order effects of a displacement of beamsteering element 158 on the optical path length of the measurement beamrelative to the optical path length of the reference beam will be firstorder in the magnitude of the displacement of beam steering element 158,first order in the magnitude of a change of orientation of themeasurement object mirror 170, and first order in some uncompensatederrors in the figure of one or more surfaces of elements of theinterferometer.

The interferometer and the beam steering assembly are configured suchthat changes in the orientations of the object mirror (about the centerof a measurement beam spot on the object mirror) and/or of beam steeringelement 158 do not effect in first order the optical path length of themeasurement beam relative to the optical path length of the referencebeam; the residual second order effect is proportional the difference inthe path lengths of the measurement and reference beams and proportionalto a factor which is second order in the angular change in orientationof the object mirror about an orientation corresponding to themeasurement beam spot. The magnitude of the second order effect isreduced by the single-pass of the measurement beam to the measurementobject relative to multiple-pass arrangements.

The second order effect cited in the preceding paragraph is describedwith the aid of FIG. 1f. FIG. 1f depicts in schematic form theequivalent path of the measurement beams for a first case of objectmirror at initial angular orientation and for a second case ofmeasurement object mirror 170 with a change in orientation in the planeof FIG. 1f. Measurement object mirror 170 for the second case is shownas measurement object mirror 170A rotated about point a by an angle β.The distance measured by the interferometer for case one is twice thedistance between the points a and b, the distance between the points aand b being denoted by ab. The line from point b to point c isorthogonal to the surface of measurement object mirror 170A as aconsequence of the action of the beam steering element 158 controlled ofthe servo system. Therefore the distance measured by the interferometerfor case two is twice the distance between the points b and c, thedistance between the points b and c being denoted by bc. The distance2bc is related to the distance 2ab as:

2bc=2ab cos β  (1)

The cosine factor in equation (1) differs from the value of one by aterm that is second order β, i.e. β²/2.

Another feature of the first embodiment is that there will be no lateralshear of either the measurement or reference beams and beam componentsin at least first order produced by a translation of beam steeringelement 158 in the nominal direction of propagation of beams exitingbeam steering element 158; the directions of propagation of thereference and measurement beams entering and exiting beam steeringelement 158 are substantially parallel.

A notable feature of the first embodiment is that the interferometer andthe beam steering assembly is of the noninverting type, i.e.corresponding points in the wavefront amplitudes of the reference andmeasurement beam components of input beam 110 map to substantiallysuperimposed points in the wavefront amplitudes of the reference andmeasurement beam components of output beam 130 and of the reference andmeasurement beam components of output beam 132. Associated with thislatter feature is a concomitant feature that the angle between thereference and measurement beam components of beam 130 and the anglebetween the reference and measurement beam components of beam 132 arenot effected in first and higher orders by changes in the direction ofpropagation of input beam 110.

It will be apparent to those skilled in the art that as a consequence ofthe concomitant feature cited in the last paragraph, detector 188 canperform its function in the servo system by detecting changes in alateral position in one or two orthogonal directions of the measurementbeam component of the first output beam 130 if the direction ofpropagation of the input beam 110 is fixed without departing from eitherthe spirit or the scope of the present invention. To change theproperties of detector 188 from the detection of a change in directionof propagation of a particular beam in one plane to the detection of alateral shear in the one plane, a portion of the particular beam issplit off by a nonpolarizing beam splitter and transmitted withoutpassing through a focusing lens directly to a detector such as a dualcell detector, the dual cell detector being orientated to detect thelateral shear. The dual cell detector is sensitive to a lateral shear ofthe particular beam but not sensitive to a change at the dual celldetector in the direction of propagation of the particular beam. Tochange the properties of detector 188 from the detection of changes indirection of propagation in two orthogonal planes of a particular beamto the detection of a lateral shear in two orthogonal directions, theparticular beam is transmitted without passing through a focusing lensdirectly to a detector such as a quad cell detector. The quad celldetector is sensitive to a lateral shear of the particular beam but notsensitive to a change at the quad cell detector in the direction ofpropagation of the particular beam.

It will further be evident to those skilled in the art that in anapplication of the first embodiment to a lithographic stepper/scanner,the measurement object mirror can be attached to a wafer stage and thereference mirror can be attached to a lens assembly, the lens assemblyused to focus an exposure radiation beam onto a wafer, to form a columnreference without departing from the scope or spirit of the invention.

There are several additional advantages of the first embodiment. Oneadditional advantage is that the paths for the measurement beam andreference beam are highly symmetric, meaning generally that for eachsegment of the measurement beam path there is a substantially parallelcorresponding segment of the reference beam path except for the variableair path defined by the location of the measurement object mirror 170and quarter wave phase retardation plate 168. Further, the measurementbeam and the reference beam can be made to pass through the same amountof glass and air, exclusive only of the variable air path defined by thelocation of measurement object mirror 170 and quarter wave phaseretardation plate 168, by replacing with glass a section of the path ofreference beam 114 that is in air (cf. FIG. 1b). This symmetry leads tohigh thermal and mechanical stability.

Another additional advantage is that the measurement beam and referencebeam each reflect from the same flat surfaces, including the surfaces ofthe retroreflector 172, an equal number of times except for surfaces 152and 156 of rhomboidal prism 154 of the first beam splitting assembly,surfaces 162 and 184 of dove prism 182 of the second beam splittingassembly, and measurement object mirror 170 and reference mirror 178.This greatly reduces the requirements for parallelism of the variousreflecting surfaces of the first embodiment.

Another additional advantage is that the first embodiment is comprisedof easily fabricated optical components. Retroreflector 172 may be ofthe common type, either solid or hollow. The first and second beamsplitting assemblies may be fabricated from ordinary right angle prisms,dove prisms, and rhomboidal prisms. The beam steering element 158 beinga trapezoidal shape prism which may be easily fabricated, either solidor hollow. The trapezoidal shape prism may be fabricated in particularby truncating an equilateral prism.

Another additional advantage is that there are very few opportunitiesfor ghost reflections, that is spurious reflections, particularly withregard to spurious multiple passing of beams to measurement objectmirror 170. This characteristic reduces the possibility for generationof cyclic errors in the relative phase of the measurement and referencebeams measured by the inventive apparatus, cyclic errors that are commonto prior-art plane-mirror interferometers.

Another feature of the first embodiment is that beam steering element158 effects changes in the ellipticity of reflected beams that is of theorder of or less than the changes in orientation of beam steeringelement 158. The effect on the cyclic errors in the measured relativephase difference between the measurement and reference beams istypically a third order effect; an example of one effect is an effectfirst order in the size of the initial value of the cyclic error andsecond order in the change in orientation of beam steering element 158about a preselected orientation.

Another feature of the first embodiment is a simple alignment procedure.The existence of a relative angle detector in detector 188 simplifiesalignment of the apparatus as follows. By blocking the measurement beambetween quarter wave phase retardation plate 168 and measurement objectmirror 170, electrical error signal 188 becomes indicative of thealignment of entrance beam 110, assuming that all other alignments arecorrect. Viewing this signal through additional display means (notshown) provides information on how best to align entrance beam 110.

As a further improvement upon the alignment procedure, the first andsecond beam splitting assemblies may be provided with additional servocontrol and additional orientation/translation transducers (not shown)to automatically align the apparatus of the first embodiment byadjustment of the angular orientation and/or position of the first andsecond beam splitting assemblies. As an alternative to the furtherimprovement upon the alignment procedure, reference mirror 178 may beprovided with additional servo control and additional orientationtransducers (not shown) to automatically align the apparatus of thefirst embodiment by adjustment of the angular orientation of referencemirror 178.

All surfaces of elements of the first embodiment through which beams aretransmitted are antireflection coated to maintain a high transmissionefficiency for the interferometer.

It will be evident to those skilled in the art that depending upon theend use application, the first embodiment may be implemented tocompensate for changes in orientation of the measurement object mirrorabout only one axis of rotation instead of two orthogonal axes ofrotation without departing either from the scope or spirit of thepresent invention.

FIGS. 2a-2 f depict, in schematic form, the second embodiment of thepresent invention for measuring a linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Thesecond embodiment is from the first class of embodiments and variantsthereof operating in the first mode of operation for changes inorientation of the object about two orthogonal axes. The secondembodiment uses beam polarization encoding for beam identification andcontrol of beam paths in certain steps relating to a steering elementwhere the first embodiment uses spatial separation of beams for keepingtract of the identity of reference and measurement beams in othercertain steps relating to the steering element.

The second embodiment comprises light beam 210, a light beam steeringassembly comprising beam steering element 270 andorientation/translation transducer 271, a first, second, third, andfourth beam splitting assemblies BAS1, BAS2, BAS3, and BAS4,respectively, retroreflector 293, measurement object mirror 284comprised of a single reflecting surface, and reference beam mirror 265comprised of a single reflecting surface. Prisms 250, 254, 258, 262,267, 269, 272, and 296, half-wave phase retardation plates 260 and 294,and quarter wave phase retardation plate 268 comprise the first beamsplitting assembly BAS1 wherein interface surfaces 252, 256, 266, and298 are polarizing beam splitting interfaces, and surfaces 264 and 274are reflecting surfaces (cf. FIG. 2b). Prisms 276 and 277 comprise thesecond beam splitting assembly BAS2 wherein interface 280 is apolarizing beam splitting interface and surface 278 is a reflectingsurface (cf. FIG. 2e). Prisms 286 and 292 comprise the third beamsplitting assembly BAS3 wherein interface 288 is a polarizing beamsplitting interface (cf. FIG. 2d). Prisms 251 and 255 comprise thefourth beam splitting assembly BAS4 wherein interface 253 is anonpolarizing beam splitting interface (cf. FIG. 2a).

The second embodiment is adapted for use as a heterodyne interferometersystem. The descriptions of light beam 210 and the source of beam 210are the same as the corresponding descriptions given for light beam 110and the source of beam 110 of the first embodiment.

Beam 210 as illustrated in FIG. 2b, impinges on BAS1 with a portion ofbeam 210 reflected by interface 252 as beam 211 and beam 211 isreflected by interface 256 as measurement beam 213. Beam 213 ispolarized orthogonal to the plane of FIG. 2b. A second portion of beam210 is transmitted by interface 252 as reference beam 212 polarized inthe plane of FIG. 2b.

Beam 213 is transmitted by quarter wave phase retardation plate 268 as acircularly polarized beam, reflected by beam steering element 270 as abeam with opposite-handed circular polarization, and subsequentlytransmitted by quarter wave phase retardation plate 268 as beam 215linearly polarized in the plane of FIG. 2a (cf. FIG. 2b). Beam steeringelement 270 is comprised of a single reflecting surface. Beam 215 istransmitted by polarizing interface 256 as beam 217 of BAS1. Beam 217 isreflected by reflecting interface 274 and exits BAS1 as beam 219.

Beam 219 enters BAS2 (cf. FIG. 2e) and is reflected by reflectingsurface 278 as beam 221 and beam 221 is reflected by polarizing beamsplitting interface 280 as beam 223.

Beam 223, being polarized orthogonal to the plane of FIG. 2e, istransmitted by quarter wave phase retardation plate 282 as a circularlypolarized beam, reflected by measurement object mirror 284 as a beamwith opposite-handed circular polarization, and subsequently transmittedby quarter wave phase retardation plate 282 as beam 225 linearlypolarized in the plane of FIG. 2e and orthogonal to the plane of FIG.2a.

Beam 225 enters BAS2 and is transmitted by polarizing beam splittinginterface 280 as beam 227. Beam 227 exits BAS2, enters BAS3 (cf. FIG.2d), and is reflected by the polarizing beam splitting interface 288 asbeam 229. Beam 229, being polarized orthogonal to the plane of FIG. 2d,is transmitted by quarter wave phase retardation plate 290 as acircularly polarized beam, reflected by steering mirror 270 as a beamwith opposite-handed circular polarization, and subsequently transmittedby quarter wave phase retardation plate 290 as beam 231 linearlypolarized in the plane of FIG. 2d. Beam 231 is transmitted by polarizingbeam splitting interface 288 as beam 233.

Beam 233 enters retroreflector 293 (cf. FIG. 2d) and is retroreflectedas beam 235 (cf. FIG. 2a). Beams 233 and 235 are shown in FIG. 2f asbeams entering and exiting, respectively, retroreflector 293.

Reference beam 212, polarized in the plane of FIG. 2b, is transmitted byhalf-wave phase retardation plate 260 as beam 214, half-wave phaseretardation plate 260 being orientated so the beam 214 is polarizedorthogonal to the plane of FIG. 2b. Beam 214 is reflected by reflectingsurface 264 as beam 216 and beam 216 is reflected by polarizing beamsplitting interface 266 as beam 218. Beam 218, being polarizedorthogonal to the plane of FIG. 2b, is transmitted by quarter wave phaseretardation plate 268 as a circularly polarized beam, reflected bysteering mirror 270 as a beam with opposite-handed circularpolarization, and subsequently transmitted by quarter wave phaseretardation plate 268 as beam 220 linearly polarized in the plane ofFIG. 2b. Beam 220 is transmitted by polarizing beam splitting interface266 as beam 222.

Beam 222 enters retroreflector 293 (cf. FIG. 2a) and is retroreflectedas beam 224 (cf. FIG. 2d). In FIG. 2f, beams 222 and 224 are shown asbeams entering and exiting, respectively, retroreflector 293. Beam 224enters BAS3 and is transmitted by polarizing beam splitting interface288 as beam 226 (cf. FIG. 2d). Beam 226, being polarized in the plane ofFIG. 2d, is transmitted by quarter wave phase retardation plate 290 as acircularly polarized beam, reflected by beam steering mirror 270 as abeam with opposite-handed circular polarization, and subsequentlytransmitted by quarter wave phase retardation plate 290 as beam 228linearly polarized orthogonal to the plane of FIG. 2d. Beam 228 isreflected by polarizing beam splitting interface 288 as beam 230.

Beam 230 enters BAS2 (cf. FIG. 2d) and is transmitted by polarizing beamsplitting interface 280 as beam 232. The projections of beam 223 and 232are superimposed in the plane of FIG. 2e. Beam 232, being polarizedorthogonal to the plane of FIG. 2d, is transmitted by quarter wave phaseretardation plate 282 as a circularly polarized beam, reflected byreference mirror 265 as a beam with opposite-handed circularpolarization, and subsequently transmitted by quarter wave phaseretardation plate 282 as beam 234 linearly polarized in the plane ofFIG. 2d.

Beam 234 enters BAS2, reflected by polarizing beam splitting interface280, and subsequently reflected by reflecting surface 278 as beam 238(cf. FIG. 2a).

Beam 235 enters BAS1 and, being polarized in the plane of FIG. 2b, istransmitted by half-wave phase retardation plate 294 as beam 237.Half-wave phase retardation plate 294 is orientated so that beam 237 ispolarized orthogonal to the plane of FIG. 2b. Beam 237 is reflected bypolarizing beam splitting interface 298 as one component of output beam240. Beam 238 is transmitted by polarizing beam splitter interface 298as a second component of output beam 240.

As shown in FIG. 2a, beam 240 enters BAS4 and a first portion of outputbeam 240 is transmitted by nonpolarizing beam splitting interface 253 asthe first output beam 241. First output beam 241 impinges on detector257 to produce an electrical signal 244 related to any difference in thedirections of propagation of measurement and reference beam componentsin first output beam 241 in two orthogonal planes.

A second portion of beam 240 is reflected by nonpolarizing beamsplitting interface 253 as the second output beam 242. Beam 242 istransmitted by a polarizer 259 orientated to produce a mixed opticalbeam 243. Beam 243 impinges on detector 261 to generate electricalsignal 245 related to the difference in phase of the measurement andreference beam components of mixed beam 243. Electrical signal 245 istransmitted to electronic processor and computer 263 for subsequentprocessing for information about the difference in phase of themeasurement and reference beam components of mixed beam 243.

Electrical signal 244 is transmitted to orientation/translationtransducer 271. Information contained in signal 244 is used as the errorsignal in a servo system to alter the orientation of beam steeringmirror 270 by orientation/translation transducer 271 so as to maintainthe direction of propagation of measurement beam 237 substantiallyconstant independent of changes of orientation of object mirror 284.

The remaining description of the second embodiment is the same ascorresponding portions of the description of the first embodiment exceptwith regard to generation of cyclic errors. The additional phaseretardation plates in the second embodiment in relation to the firstembodiment increase the number of possibilities for the generation ofcyclic errors in the relative phase measured between the reference andmeasurement beam components of the output beam. However, the cyclicerrors generated in the second embodiment as a result of the additionalphase retardation plates are generally substantially less thancorresponding cyclic errors present in prior art plane mirrorinterferometers, a consequence principally because there is only onepass to the measurement object mirror 284 by the measurement beam in thesecond embodiment in comparison to the double pass to a measurementobject mirror by the measurement beam in many prior art plane mirrorinterferometers.

FIGS. 3a-3 f depict, in schematic form, the third embodiment of thepresent invention for measuring a linear orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Thethird embodiment is from the first class of embodiments and variantsthereof operating in the first mode of operation for changes inorientation of the object about a first axis and in the second mode ofoperation for changes in orientation of the object about a second axisorthogonal to the first axis and/or a lateral displacement of theobject, the lateral displacement being orthogonal to both the secondaxis and the linear displacement.

The interferometer system of the first and third embodiments aregenerically similar except with respect to the reflecting properties ofthe object: the reflecting properties of the object for the firstembodiment are those of a plane mirror and the reflecting properties ofthe object for the third embodiment are those of a roof prism.

The third embodiment comprises light beam 310, a light beam steeringassembly comprising beam steering element 362 andorientation/translation transducer 363, a first, second, third, andfourth beam splitting assemblies BAS1, BAS2, BAS3, and BAS4,respectively, retroreflector 368, measurement object roof prism 366, andreference beam prism 364. Rhomboidal prism 350 and prism 354 comprisethe first beam splitting assembly BAS1 wherein interface 352 is apolarizing beam splitting interface and surface 356 is a reflectingsurface (cf. FIG. 3b). Prisms 358 and 370 comprise the second beamsplitting assembly BAS2 wherein interface 360 is a reflecting interface(cf. FIG. 3a). Prisms 372 and 376 comprise the third beam splittingassembly BAS3 wherein interface 374 is a reflecting interface (cf. FIG.3d). Rhomboidal prisms 378 and 384 comprise the fourth beam splittingassembly BAS4 wherein interface 380 is a nonpolarizing beam splittinginterface and surfaces 382 and 386 are reflecting surfaces (cf. FIG.3e).

The third embodiment is adapted for use as a heterodyne interferometersystem. The descriptions of light beam 310 and the source of beam 310are the same as the corresponding descriptions given for the light beam110 and the source of beam 110 of the first embodiment.

Beam 310, as illustrated in FIG. 3a, impinges BAS1 with a portion ofbeam 310 reflected by interface 352 and reflected by surface 356 asmeasurement beam 311. Beam 311 is polarized orthogonal to the plane ofFIG. 3a. A second portion of beam 310 is transmitted by interface 352 asreference beam 312 polarized in the plane of FIG. 3a.

Beam 311, as illustrated in FIG. 3b, impinges on BAS2 and is reflectedby interface 360 as measurement beam 313. Beam 313 is polarized in theplane of FIG. 3b.

Beam 313 enters beam steering element 362 (cf. FIG. 3b) and exits beamsteering element 362 as beam 315 (cf. FIG. 3d) displaced laterally frombeam 313 and propagating in a direction nominally opposite to thedirection of propagation of beam 313. The projections of beam 313 and ofbeam 315 on the plane of FIG. 3b are superimposed. In beam steeringelement 362, beam 313 is reflected in a sequence, e.g., by totalinternal reflection, by each of the surfaces 362A, 362B, and 362C (cf.FIG. 3c which presents an side view of the interferometer depicted inFIGS. 3b and 3 d). Beam steering element 362 and orientation/translationtransducer 363 comprise the beam steering assembly that redirectsmeasurement beam 313. Orientation/translation transducer 363 preferablyalters the orientation of beam steering element 362 about a nodal pointof retroreflector 368 for a change in orientation in the plane of FIG.3c, changes in orientation of beam steering element 362 about a nodalpoint of retroreflector 368 producing substantially reduced lateralshear effects experienced by the reference beam, and/or the locationbeam steering element 362 in the nominal direction of propagation ofbeam 313. The orientation of beam steering element 362 is controlled byelectronic signal 340. The location of beam steering element 362 iscontrolled by electronic processor and computer 394 through electronicsignal 344.

Beam 315 is transmitted by dove prism 364 as beam 317 (cf. FIG. 3d).Beam 317 is reflected by measurement object roof prism 366 as beam 319linearly polarized in the plane of FIG. 3d. The projections of beam 317and beam 319 on the plane of FIG. 3c are superimposed. Measurementobject roof prism 366 may be of either solid or hollow construction.Beam 319 is transmitted by dove prism 364 as beam 321.

Beam 321 enters beam steering element 362 (cf. FIG. 3d) and exits beamsteering element 362 as beam 323 (cf. FIG. 3b) propagating in adirection nominally opposite to the direction of propagation of beam321. Beam 323 enters retroreflector 368 (cf. FIG. 3b) and isretroreflected as beam 325. An end view of retroreflector 368 and doveprism 364 is depicted in FIG. 3f. Beam 325 enters BAS2 and is reflectedby interface 360 as beam 327.

Reference beam 312 enters BAS3 and is reflected by interface 374 as beam314 (cf. FIG. 3d). Beam 314 enters beam steering element 362 (cf. FIG.3d) and exits beam steering element 362 as beam 316 (cf. FIGS. 3b and 3d) propagating in a direction nominally opposite to the direction ofpropagation of beam 314. The projections of beam 316 and of beam 314 onthe plane of FIG. 3b are superimposed. The projections of beam 316 andof beam 313 on the plane of FIG. 3c are superimposed. Beam 316 entersretroreflector 368 and is retroreflected as beam 318 (cf. FIG. 3b). Beam318 enters beam steering element 362 (cf. FIG. 3b) and exits beamsteering element 362 as beam 320 (cf. FIG. 3d) propagating in adirection nominally opposite to the direction of propagation of beam318.

Measurement beams 323 and 325 and reference beams 316 and 318 lie in thesame top beam plane (cf. FIG. 3b). Measurement beams 315 and 321 andreference beams 314 and 320 lie in the same bottom beam plane (cf. FIG.3d).

Beam 320 enters dove prism 364 and is reflected by surfaces 364A and364B as beam 324. Dove prism 364 functions as a roof prism inredirecting beam 320 as beam 324 in the same manner as measurementobject roof prism 366 functions in redirecting beam 317 as beam 319.Beam 324, orthogonally polarized to the plane of FIG. 3d, enters BAS3and is reflected by interface 374 as beam 326.

Beam 327 enters BAS4 (cf. FIG. 3e) and a first portion of beam 327 isreflected by nonpolarizing beam splitting interface 380 as one componentof a first output beam 330. Beam 326 enters BAS4 and is reflected bysurface 386 as beam 328. A first portion of beam 328 is transmitted bybeam splitting interface 380 as a second component of the first outputbeam 330. First output beam 330 is reflected by surface 382 as outputbeam 332. Output beam 332 impinges on detector 390 to produce anelectrical signal 340 with one component related to any lateral shear ofthe measurement beam component of output beam 332 in the plane of FIG.3d, i.e. orthogonal to the plane of FIG. 3e, and a second component torelated to any difference in the directions of propagation ofmeasurement and reference beam components in output beam 332 orthogonalto the plane of FIG. 3d, i.e. in the plane of FIG. 3e.

A second portion of beam 327 is transmitted by nonpolarizing beamsplitting interface 380 as one component of a second output beam 334. Asecond portion of beam 328 is reflected by nonpolarizing beam splittinginterface 380 as a second component of the second output beam 334. Beam334 is transmitted by a polarizer 388 that is orientated to produce amixed optical beam 336. Beam 336 impinges on detector 392 to generateelectrical signal 342 related to the difference in phase of themeasurement and reference beam components of mixed beam 336. Electricalsignal 342 is transmitted to electronic processor and computer 394 forsubsequent processing for information about the difference in phase ofthe measurement and reference beam components of mixed beam 336.

Electrical signal 340 is transmitted to orientation/translationtransducer 363. Information contained in signal 340 is used as the errorsignals in a servo system to alter the orientation of beam steeringelement 362 by orientation/translation transducer 363 so as to maintainboth the lateral shear of beam 327 in the plane of FIG. 3b and thedirection of propagation of measurement beam 327 in a plane orthogonalto the plane of FIG. 3b substantially constant independent of changes oforientation of measurement object roof prism 366. Under the conditionthat the physical path length of the measurement beam from the firstredirection by the beam steering element 362 to measurement object roofprism 366 is equal to the physical path length of the measurement beamfrom measurement object roof prism 366 to the second redirection by beamsteering element 362, there will be no lateral shear of beam 327orthogonal to the plane of FIG. 3b when changes in orientation ofmeasurement object roof prism 366 are compensated by changes inorientation of beam steering mirror 362. This condition with respect tophysical path lengths will be referenced as the third embodimentcondition.

Compliance with the third embodiment condition eliminates the lateralshear of measurement beam 323 at beam steering element 362 when changesin orientation of measurement object roof prism 366 are compensated bychanges in orientation of beam steering element 362. However, there willbe introduced lateral shear of reference beam 316 when changes inorientation of measurement object roof prism 366 are compensated bychanges in orientation of beam steering element 362. Although a lateralshear will be introduced in reference beam 316, the magnitude of thelateral shear is significantly reduced in relation to the lateral shearintroduced in measurement beam components of prior art plane mirrorinterferometers due to changes in orientation of the object mirror, thereduction factor being proportional to the ratio of the physical lengthof the reference leg to the physical length of the measurement leg of aninterferometer.

The remaining description of the third embodiment is the same ascorresponding portions of the description given for the firstembodiment.

A first variant of the third embodiment of the present invention isdescribed for measuring the linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Thefirst variant of the third embodiment is from the first class ofembodiments and variants thereof operating in the second mode ofoperation for changes in orientation of the object about two orthogonalaxes and/or for a lateral displacement of the object, the lateraldisplacement being orthogonal to the linear displacement.

The interferometer system and at least one beam steering assembly of thefirst variant of the third embodiment comprise the same elements as theinterferometer system and at least one beam steering assembly of thethird embodiment except for the object element, the detector of acertain optical beam, a signal produced by the detector of the certainoptical beam, and a respective servo system.

The measurement object element of the third embodiment, roof prism 366,is replaced by a measurement object retroreflector 366A (not shown in afigure) in the first variant of the third embodiment. As a consequence,the relative directions of propagation of the reference and measurementbeam components of output beam 332A, the output beam of the firstvariant of the third embodiment corresponding to output beam 332 of thethird embodiment, is substantially independent of changes in orientationof the beam steering element 362 of the first variant of the thirdembodiment. However a lateral translation of measurement objectretroreflector 366A in a plane orthogonal to the direction of the lineardisplacement and/or a change in orientation of measurement objectretroreflector 366A will generate a lateral displacement of measurementbeam component of output beam 332A.

Detector 390A of the first variant of the third embodiment,corresponding to detector 390 of the third embodiment, detects thelateral displacement or shear of the measurement beam component ofoutput beam 332A in two orthogonal directions and generates electricalsignal 340A, the electrical signal corresponding to electrical signal340 of the third embodiment.

Electrical signal 340A is transmitted to orientation/translationtransducer 363 of the first variant of the third embodiment. Informationcontained in signal 340A is used as the error signal in a servo systemto alter the orientation of beam steering element 362 byorientation/translation transducer 363 so as to maintain the laterallocation of measurement beam component of output beam 332A substantiallyconstant independent of a lateral translation of measurement objectretroreflector 366A in a plane orthogonal to the direction of the lineardisplacement and/or substantially independent of changes in orientationof measurement object retroreflector 366A about two orthogonal axes.

The remaining description of the first variant of the third embodimentis the same as corresponding portions of the description given for thethird embodiment.

FIGS. 4a-4 f depict, in schematic form, the fourth embodiment of thepresent invention for measuring a linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a being orthogonal to the lineardisplacement. The fourth embodiment is from the first class ofembodiments and variants thereof operating in the first mode ofoperation for changes in orientation of the object about a first axisand in the second mode of operation for changes in orientation of theobject about a second axis orthogonal to the first axis and/or a lateraldisplacement of the object, the lateral displacement being orthogonal toboth the second axis and the linear displacement.

The interferometer system of the third and fourth embodiments aregenerically similar except with respect to the number of passes themeasurement beam makes to the object and the associated number of passesmade by the reference beam to a reference mirror: the measurement(reference) beam of the fourth embodiment traverses certain sections ofthe measurement (reference) path in two opposite directions ofpropagation where in the third embodiment, the corresponding measurement(reference) beam transits the certain sections in only one direction ofpropagation. Efficient separation of an output measurement (reference)beam component from an input measurement (reference) beam component inthe multiple pass configuration of the fourth embodiment is effected byuse of polarization encoding and polarizing beam splitters.

Many elements of the third and fourth embodiments perform like functionsand these elements are indicated by a numeric numbering scheme whereinthe numeric number of elements performing like functions for the fourthembodiment are equal to the numeric number of corresponding elements ofthe third embodiment incremented by 100. The certain sections of thereference and measurement beam paths are indicated by the numericnumbering scheme wherein the numeric number of the certain section ofthe reference or measurement path of the fourth embodiment having thesame description as a corresponding path section of the third embodimentis equal to the numeric number of corresponding path section of thethird embodiment incremented by 100.

For the certain sections of the measurement (reference) beam path forthe fourth embodiment, there is a measurement (reference) beamtraversing the certain sections in a reverse direction of propagation,the nonreverse or forward direction of propagation being associated witha first pass to the measurement object roof prism 466 (reference beamdove prism 464) and the reverse direction of propagation beingassociated with a second pass to the measurement object roof prism 466(reference beam dove prism 464). The description of the additionalmeasurement (reference) beam traversing the certain sections in areverse direction are otherwise the same as the corresponding portionsof the description given associated measurement (reference) beamtraversing in a nonreverse direction of propagation for the thirdembodiment, the alphanumeric number of a measurement (reference) beamtraversing in a reverse direction being equal to the numeric number ofthe associated measurement (reference) beam traversing in a nonreversedirection augmented with the suffix R.

The fourth embodiment comprises light beam 410, a light beam steeringassembly comprising beam steering element 462 andorientation/translation transducer 463, a first, second, third, andfourth beam splitting assemblies, retroreflector 468, measurement objectelement 466 comprising a roof prism, reference beam element 464comprising a dove prism, mirror 461, and quarter wave phase retardationplate 457. Rhomboidal prism 450 and prism 454 comprise the first beamsplitting assembly BAS1 wherein interface 452 is a polarizing beamsplitting interface and surface 456 is a reflecting surface (cf. FIG.4a). Prisms 458 and 470 (cf. FIG. 4b) comprise the second beam splittingassembly BAS2 wherein interface 460 is a reflecting interface. Prisms472 and 476 (cf. FIG. 4d) comprise the third beam splitting assemblyBAS3 wherein interface 474 is a reflecting interface. Prisms 451, 453,and 465 and rhomboidal prism 467 comprise the fourth beam splittingassembly BAS4 (cf. FIG. 4e) wherein interface 455 is a nonpolarizingbeam splitting interface, interface 469 is a polarizing beam splittinginterface, and surface 471 is a reflecting surface.

The descriptions of the generation of measurement beam 427 and referencebeam 426 is the same as the descriptions given for correspondingdescriptions of the generation of measurement beam 327 and referencebeam 326 of the third embodiment. As shown in FIG. 4e, beam 427 entersthe fourth beam splitting assembly and a first portion of beam 427 istransmitted by nonpolarizing beam splitting interface 455 as measurementbeam component 401 polarized orthogonal to the plane of FIG. 4e. Asecond portion of beam 427 is reflected by nonpolarizing beam splittinginterface 455 as measurement beam component 403 polarized orthogonal tothe plane of FIG. 4 e. Beam 426 enters BAS4 and a first portion of beam426 is transmitted by nonpolarizing beam splitting interface 455 asreference beam component 400 polarized in the plane of FIG. 4e. A secondportion of beam 426 is reflected by nonpolarizing beam splittinginterface 455 as measurement beam component 402 polarized in the planeof FIG. 4e.

Beam 401 is reflected by polarizing beam splitting interface 469 as onecomponent of a first output beam 406. Beam 402 is reflected byreflecting surface 404 and beam 404 is transmitted by polarizing beamsplitting interface 469 as a second component of the first output beam406. First output beam 406 impinges on detector 496 to produce anelectrical signal 441 with a first component related to any lateralshear of the measurement beam component of output beam 406 produced by adisplacement of measurement object roof prism 466 in the plane of FIG.4d and nominally orthogonal to the direction of propagation of beam 417and/or a change in orientation of measurement object roof prism 466about an axis perpendicular to the plane of FIG. 4d and with a secondcomponent related to any difference in the directions of propagation ofmeasurement and reference beam components in output beam 406 produced bya change in orientation of the measurement object roof prism 466 aboutan axis in the plane of FIG. 4d and nominally orthogonal to thedirection of propagation of beam 417.

In FIG. 4e, beam 403, polarized orthogonal to the plane of FIG. 4d, istransmitted by quarter wave phase retardation plate 457 as a circularlypolarized beam, reflected by mirror 461 as a beam with opposite-handedcircular polarization, and subsequently transmitted by phase retardationplate 457 as beam 405 linearly polarized in the plane of FIG. 4e. Beam405 enters BAS4 and a portion of beam 405 is reflected by nonpolarizingbeam splitting interface 455 as beam 427R. Beam 427R has polarizationorthogonal to the polarization of beam 427.

Beam 402, polarized in the plane of FIG. 4e, is transmitted by quarterwave phase retardation plate 457 as a circularly polarized beam,reflected by mirror 461 as a beam with opposite-handed circularpolarization, and subsequently transmitted by phase retardation plate457 as beam 408 polarized orthogonal to the plane of FIG. 4e. Beam 408enters BAS4 and a portion of beam 408 is reflected by nonpolarizing beamsplitting interface 455 as beam 426R. Beam 426R has polarizationorthogonal to the polarization of beam 426.

Beam 427R is substantially coextensive with beam 427 and beam 427R has adirection of propagation substantially opposite to the direction ofpropagation of 427. As a consequence, beam 427R substantially retracesthe pass through portions of the interferometer, the pass leading to thegeneration of beam 427, to generate beam 411R. The retrace throughportions of the interferometer includes in particular a pass to themeasurement object roof prism 466. Beam 411R is substantiallycoextensive with beam 411 and beam 411R has a direction of propagationsubstantially opposite to the direction of propagation of beam 411.

Beam 426R is substantially coextensive with beam 426 and beam 426R has adirection of propagation substantially opposite to the direction ofpropagation of 426. As a consequence, beam 426R substantially retracesthe pass through portions of the interferometer, the pass leading to thegeneration of beam 426, to generate beam 412R. The retrace throughportions of the interferometer includes a pass to the reference doveprism 464. Beam 412R is substantially coextensive with beam 412 and beam412R has a direction of propagation substantially opposite to thedirection of propagation of 412.

As shown in FIG. 4a, beam 411R being polarized in the plane of FIG. 4aenters the first beam splitting assembly to be reflected by reflectingsurface 456 and transmitted by polarizing beam splitting interface 452as a first component of a second output beam 446. Beam 412R beingpolarized orthogonal to the plane of FIG. 4a enters the first beamsplitting assembly to be reflected by polarizing beam splittinginterface 452 as a second component of the second output beam 446.

Beam 446 is transmitted by a polarizer 497 orientated to produce a mixedoptical beam 448. Beam 448 impinges on detector 498 to generateelectrical signal 443 related to the difference in phase of themeasurement and reference beam components of mixed beam 448. Electricalsignal 443 is transmitted to electronic processor and computer 499 forsubsequent processing for information about the difference in phase ofthe measurement and reference beam components of mixed beam 448.

The significant feature of the fourth embodiment is the generation of adouble pass to the measurement object roof prism 466 over the sameoptical path normally traversed in a single pass with prior artinterferometry.

The remaining description of the fourth embodiment is the same ascorresponding portions of the description given for the thirdembodiment.

A first variant of the fourth embodiment is described wherein thegeneration of the electrical signals used in the servo system used tocontrol the beam steering element 462 differs from that used in thefourth embodiment. In the first variant of the fourth embodiment,detector 496 generates only the first component of electrical signal 441related to any lateral shear of the measurement beam component of outputbeam 406 produced by a displacement of measurement object roof prism 466in the plane of FIG. 4d and nominally orthogonal to the direction ofpropagation of beam 417 and/or a change in orientation of measurementobject roof prism 466 about an axis perpendicular to the plane of FIG.4d and a third detector 498A (not shown in a figure) is added togenerate an electrical signal 441A to serve the function of the secondcomponent of signal 441 of the fourth embodiment related to anydifference in the directions of propagation of measurement and referencebeam components in output beam 406 produced by a change in orientationof the measurement object roof prism 466 about an axis in the plane ofFIG. 4d and nominally orthogonal to the direction of propagation of beam417.

The third detector 498A detects a first portion of the second outputbeam 446 reflected by a nonpolarizing beam splitter inserted in the pathof beam 446, a second portion of second output beam 446 beingtransmitted by inserted beam splitter, transmitted by polarizer 497orientated to produce a mixed beam, and detected by detector 499.

The advantage of the first variant of the fourth embodiment is that thefirst portion of the second output beam 446 has a difference in thedirections of propagation of its measurement and reference beamcomponents that is twice the difference in the directions of propagationof the measurement and reference beam components in output beam 406produced by a change in orientation of the object roof prism 466 aboutan axis in the plane of FIG. 4d and nominally orthogonal to thedirection of propagation of beam 417. In addition, the measurement andreference beam components of the first portion of the second output beam446 have substantially no lateral shear produced by a displacement ofmeasurement object roof prism 466 in the plane of FIG. 4d and nominallyorthogonal to the direction of propagation of beam 417 and/or a changein orientation of measurement object roof prism 466 about an axisperpendicular to the plane of FIG. 4d.

A second variant of the fourth embodiment of the present invention isdescribed for measuring the linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Thesecond variant of the fourth embodiment is from the first class ofembodiments and variants thereof operating in the second mode ofoperation for changes in orientation of the object about two orthogonalaxes and/or for a lateral displacement of the object, the lateraldisplacement being orthogonal to the linear displacement.

The interferometer system and at least one beam steering assembly of thesecond variant of the fourth embodiment comprise the same elements asthe interferometer system and at least one beam steering assembly of thefourth embodiment except for the object element, the detector of acertain optical beam, a signal produced by the detector of the certainoptical beam, and a respective servo systems.

The measurement object element of the fourth embodiment, measurementobject roof prism 466, is replaced by an measurement objectretroreflector 466B (not shown in a figure) in the second variant of thefourth embodiment. As a consequence, the relative directions ofpropagation of the reference and measurement beam components of outputbeam 401B, the output beam of the second variant of the fourthembodiment corresponding to output beam 401 of the fourth embodiment,are substantially independent of changes in orientation of the beamsteering element 462 of the second variant of the fourth embodiment.However a lateral translation of measurement object retroreflector 466Bin a plane orthogonal to the direction of the linear displacement willgenerate a lateral displacement of measurement beam component of outputbeam 401B.

Detector 496B of the second variant of the fourth embodiment,corresponding to detector 496 of the fourth embodiment, detects thelateral displacement or shear of the measurement beam component ofoutput beam 401B in two orthogonal directions and generates electricalsignal 441B, the electrical signal corresponding to electrical signal441 of the fourth embodiment. Electrical signal 441B is comprised of twocomponents, one component related to any lateral shear of themeasurement beam component of output beam 406 produced by a displacementof measurement object retroreflector 466B in the plane of FIG. 4d andnominally orthogonal to the direction of propagation of beam 417 and/ora change in orientation of measurement object retroreflector 466B aboutan axis perpendicular to the plane of FIG. 4d and a second componentrelated to any lateral shear of the measurement beam component of outputbeam 406 produced by a displacement of measurement object retroreflector466B perpendicular to the plane of FIG. 4d and nominally orthogonal tothe direction of propagation of beam 417 and/or a change in orientationof measurement object retroreflector 466B about an axis parallel to theplane of FIG. 4d.

Electrical signal 441B is transmitted to orientation/translationtransducer 463 of the second variant of the fourth embodiment.Information contained in signal 441B is used as error signals in a servosystem to alter the orientation of beam steering element 462 byorientation/translation transducer 463 so as to maintain the laterallocation of measurement beam component of output beam 401B substantiallyconstant independent of a lateral translation of measurement objectretroreflector 466B in a plane orthogonal to the direction of the lineardisplacement and/or substantially independent of changes in orientationof measurement object retroreflector 466B about two orthogonal axes.

The remaining description of the second variant of the fourth embodimentis the same as corresponding portions of the description given for thefourth embodiment.

FIG. 5 depicts, in schematic form, the fifth embodiment of the presentinvention for measuring a linear displacement of an object substantiallyindependent of the orientation of the object and substantiallyindependent of a lateral displacement of the object, the lateraldisplacement being orthogonal to the linear displacement. The firstembodiment is from the first class of embodiments and variants thereofof the present invention operating in the first mode of operation forchanges in orientation of the object about two orthogonal axes.

The embodiment is comprised principally of a beam division andrecombination optic 530, beam steering mirror 540 attached to aorientation/translation transducer 542, a relative beam angle sensorassembly 550, a reference mirror 536, two retroreflectors 531 and 533, aphotodetector 534, two quarter wave retardation plates 538 and 539, anda measurement object mirror 590. Retroreflectors 531 and 533 aretruncated so that a beam passing centrally through either one is notretroreflected.

The fifth embodiment is adapted for use with a heterodyne interferometersystem. The description of the source of beam 510 and of beam 510 is thesame as the description of the source of beam 110 and beam 110 given forthe first embodiment. Entrance beam 510 impinges upon a polarizing beamsplitter surface 570 of beam division and recombination optic 530.Polarizing beam splitter surface 570 separates the orthogonallypolarized components of entrance beam 510 into a measurement beam 512and a reference beam 520. Measurement beam 512 initially has a linearpolarization perpendicular to the plane of the FIG. 5. Reference beam520 initially has a linear polarization in the plane of FIG. 5.Polarizing beam splitter surface 570 reflects light polarized orthogonalto the plane of FIG. 5 and transmits light polarized in the plane ofFIG. 5.

Measurement beam 512 initially traveling from left to right in FIG. 5after a first reflection from polarizing beam splitter surface 570reverses direction after reflection from retroreflector 531. Measurementbeam 512, now traveling from right to left, reflects a second time frompolarizing beam splitter surface 570 and subsequently travels upwards.Measurement beam 512 reflects a first time from adjustable mirror 540and passes centrally a first time through retroreflector 533 and then afirst time through quarter wave phase retardation plate 539. Quarterwave retardation plate 539 converts measurement beam 512 with an initiallinear polarization orthogonal to the plane of FIG. 5 , now denoted 514in the figure, to circular. Measurement beam 514 then reverses directionafter reflecting from measurement object mirror 590 and passes a secondtime through quarter wave phase retardation plate 539, resulting in anet change to linear polarization in the plane of FIG. 5. Measurementbeam 514 passes centrally a second time through retroreflector 533, thenreflects a second time from adjustable mirror 540 and travels downwards.Measurement beam 514 passes a first time through polarization beamsplitter surface 570 and reflects once from a reflective back surface571 of beam division and recombination optic 530. Measurement beam 514then passes a second time through polarization beam splitter surface 570and continues traveling from left to right as a component of exit beam516 polarized in the plane of FIG. 5.

Reference beam 520 initially traveling downwards after passing a firsttime through polarizing beam splitter surface 570 reflects once fromreflective back surface 571 of beam division and recombination optic530. Reference beam 520, now traveling from left to right, passes asecond time through polarizing beam splitter surface 570, passescentrally a first time through retroreflector 531 and then a first timethrough quarter wave phase retardation plate 538. Quarter wave phaseretardation plate 538 converts the polarization of reference beam 520,polarized in the plane of FIG. 5, to circularly polarized beam nowdenoted 522. Reference beam 522 then reverses direction after reflectingfrom reference mirror 536 and passes a second time through quarter waveretardation plate 538, resulting in a net change to linear polarizationorthogonal to the plane of FIG. 5. Reference beam 522, now travelingfrom right to left, passes centrally a second time throughretroreflector 531, then reflects a first time from polarizing beamsplitter surface 570 and travels upwards. Reference beam 522 reflects afirst time from adjustable mirror 540 and travels from left to right,then reverses direction after reflection from retroreflector 533.Reference beam 522 then reflects a second time from adjustable mirror540, travels downward, reflects a second time from polarizing beamsplitter surface 570 and continues traveling from left to right as acomponent of exit beam 516 polarized orthogonal to the plane of FIG. 5.

Exit beam 516 comprises orthogonally polarized components correspondingto measurement beam 514 and reference beam 522, respectively. Exit beam516 passes through a pick-off mirror 559 and through a mixing polarizer532 onto photodetector 534. Mixing polarizer 532, oriented atapproximately 45° with respect to the plane of the FIG. 5, combines theorthogonally polarized components of exit beam 516 into mixed beamcomprising a single linear polarization component. Photodetector 534converts the mixed beam into an electronic interference signal, e.g., byphotoelectric detection, with signal 580 being transmitted to a signalprocessing system (not shown in FIG. 5). Electronic interference signal580 has the same frequency as the optical frequency difference betweenthe orthogonal components of entrance beam 510 shifted by any Dopplershifts introduced by a relative motion of measurement mirror 590 andreference mirror 536. The phase of electronic interference signal 580varies in accordance with the difference between the optical paths ofmeasurement beam 514 and reference beam 522. Thus electronicinterference signal 580 provides a means of monitoring displacement ofmeasurement object mirror 590 with respect to reference mirror 536,using optical interference phenomena.

At the same time, a sample beam 518 from exit beam 516 reflects frompick-off mirror 559 and travels into relative beam angle sensor assembly550. Within relative beam angle sensor assembly 550 there is a polarizer558 which selects a measurement beam component out of sample beam 518.Beam 519 then travels through a lens 556 to detector 554 such as a quadcell detector, a high-speed CCD camera, or a lateral effect photodiodeor like means for determining the angular orientation of the measurementbeam component of beam 519. Information from detector 554 passes as anelectrical error signal 591 to servo control 552.

Servo control 552 sends an electronic drive signal 592 toorientation/translation transducer 542, which modifies the angularorientation of beam steering mirror 540 in response to changes in thedetected angle of measurement beam 514 as it exits beam division andrecombination optic 530. Orientation/translation transducer 542 changes,in response to signal 592, the orientation of beam steering mirror 540preferably about a nodal point of retroreflector 533, changes inorientation of beam steering mirror 542 about a nodal point of referenceretroreflector 533 producing substantially reduced lateral shear effectsexperienced by the reference beam. In normal operation, electronic servocontrol 552 maintains the angle of measurement beam 514 as it exits beamdivision and recombination optic 530 at some fixed value with respect tothe angle of reference beam 522 as it exits beam division andrecombination optic 530. Most often, it is desirable to minimize thedifference in angle between the reference beam 522 and measurement beam514 as they exit the beam division and recombination optic 530, so as tomaximize the strength of electronic interference signal 580 and minimizeerrors attributable to beam shear.

The combined action of relative beam angle sensor assembly 550, beamsteering mirror 540, and orientation/translation transducer 542 is tocompensate for changes in orientation of measurement object mirror 590.In the present embodiment, this compensation is typically achievedwithout disturbing the measured linear displacement of measurementobject mirror 590. Thus the optical path difference between measurementbeam 514 and reference beam 522 is substantially independent of changesin angle or position of beam steering mirror 540.

There are several advantages of the fifth embodiment. Firstly, the pathsfor measurement beam 514 and reference beam 522 are highly symmetric,meaning that measurement beam 514 and reference beam 522 pass throughthe same amount of glass and air, exclusive only of the variable airpath defined by the displacement of measurement object mirror 590.Measurement beam 514 and reference beam 522 also have substantiallyparallel paths, although they are spatially separated to minimizecontamination of the measurement and reference beams, one by the other,which generate cyclic errors in the measured phase of the electronicinterference signal 580. This symmetry leads to high thermal andmechanical stability.

Secondly, measurement beam 514 and reference beam 522 each reflect fromthe same flat surfaces an equal number of times, in particular, fromreflective back surface 571 and polarizing beam splitter surface 570 ofbeam division and recombination optic 530. This greatly reduces therequirements for parallelism of the two surfaces of beam division andrecombination optic 530.

Thirdly, the embodiment is comprised of easily fabricated opticalcomponents. The retroreflectors may be of the common type, either solidor hollow, with the apex ground down and polished flat. Beam divisionand recombination optic 530 may be an ordinary cube beam splitter withone edge ground flat and polished to become reflective back surface 571.Pick-off mirror 559 may be a small piece of uncoated glass, in this caseoriented to favor the component of exit beam 516 polarized orthogonal tothe plane of FIG. 5.

Fourthly, there are very few opportunities for ghost reflections, alsoknown as spurious reflections, particularly with regard to spuriousmultiple passing to measurement object mirror 590. This characteristicreduces the possibility of cyclic errors that are common to the majorityof prior-art plane-mirror interferometers.

The remaining description of the fifth embodiment is the same ascorresponding portions of the description given for the first embodimentexcept with respect to lateral shears introduced into the measurementand reference beams by a translation of the beam steering mirror 530 ina direction nominally orthogonal to the surface of the beam steeringmirror 540.

FIGS. 6a and 6 b depict in schematic form a sixth embodiment of thepresent invention for measuring a linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Thesixth embodiment is from the second class of embodiments and variantsthereof of the present invention operating in the first mode about twoorthogonal axes.

The description of light beam 610 shown in FIG. 6a and of a source oflight beam 610 is the same as the description of light beam 110 and of asource of light beam 110 given for the first embodiment. As shown inFIG. 6a, beam 610 impinges on beam splitter 650, e.g., a polarizing typebeam splitter. A portion of beam 610 is transmitted as measurement beam611 polarized in the plane of FIG. 6a and a second portion of beam 610is reflected as reference beam 612 orthogonally polarized to the planeof FIG. 6a.

In FIG. 6a, beam 611 is reflected by a first surface of beam steeringmirror 652 as beam 613. Beam steering mirror 652 and orientationtransducers 653A and 653B comprise a beam steering assembly thatredirects measurement beam 613. Beam 613 is reflected by mirror 654 andenters mirror assembly 656 as beam 615. As shown in FIG. 6b, mirrorassembly 656 is comprised of three mirrors 656A, 656B, and 656C. Theplane of FIG. 6b is orthogonal to the plane of FIG. 6a. Beam 615 isreflected by each of the three mirrors 656A, 656B, and 656C and exitsmirror assembly 656 as beam 621. Mirror assembly 656 is an imageinverter about an axis in the plane of FIG. 6a and orthogonal to thedirection of propagation of beam 615. However, the primary function ofmirror assembly 656 is to transform a change in the direction ofpropagation of beam 615 in the plane of FIG. 6b to a change in theopposite direction of propagation of beam 621 in the plane of FIG. 6band to translate a change in direction of beam 615 in the plane of FIG.6a to the same change in direction of propagation of beam 621 in theplane of FIG. 6a.

Beam 621, being polarized in the plane of FIG. 6a, is transmitted bypolarizing beam splitter 658 as beam 623. Beam 623, polarized in theplane of FIG. 6a, is transmitted by a quarter wave phase retardationplate 660 as a circularly polarized beam, reflected by measurementobject mirror 662 as a beam with opposite-handed circular polarization,and subsequently transmitted by quarter wave phase retardation plate 660as beam 627 linearly polarized perpendicular to the plane of FIG. 6a.Beam 627 is reflected by beam splitter 658 as beam 629.

Beam 629 is reflected by mirrors 664A and 664B to form beam 631. Mirrors664A and 664B together produce a constant deviation between beams 629and 631 in the plane of FIG. 6a, the same as a penta prism. Beam 631 isreflected from a second surface of beam steering mirror 652 as beam 633.A change in the orientation of measurement object mirror 662 will effecta change in direction of propagation and lateral shear of beam 633 atbeam steering mirror 652. It is possible to compensate for both of theeffects on beam 633 by an appropriate change in the orientation of beamsteering mirror 652 in conjunction with image inverter 656, beamsplitter 658, constant deviation mirror assembly comprising mirrors 664Aand 664B, and measurement object mirror 662 located in the path of themeasurement beam between the first and second reflections from beamsteering mirror 652.

Beam 633 is reflected by mirror 666 as beam 635. Beam 635 is polarizedperpendicular to the plane of FIG. 6a.

Reference beam 612 is transmitted by a quarter wave phase retardationplate 668A as a circularly polarized beam, reflected by retroreflector670, and transmitted by quarter wave phase retardation plate 668B asbeam 616 linearly polarized in the plane of FIG. 6a.

A portion of beam 635 is reflected by polarizing beam splitter 672 asone component of output beam 636. A portion of beam 616 is transmittedby polarizing beam splitter 672 as a second component of output beam636.

A portion of output beam 636 is reflected by nonpolarizing beam splitter674 as beam 638. Beam 638 impinges on detector 676 to produce anelectrical signal 644 related to any difference in the directions ofpropagation of measurement and reference beam components in beam 638.

A second portion of output beam 636 is transmitted by nonpolarizing beamsplitter 674 as beam 640. Beam 640 is transmitted by a polarizer 678orientated to produce a mixed optical beam 642. Beam 642 impinges ondetector 680 to generate electrical signal 646 related to the differencein phase of the measurement and reference beam components of mixed beam642. Electrical signal 646 is transmitted to electronic processor andcomputer 682 for subsequent processing for the information about thedifference in phase of the measurement and reference beam components ofmixed beam 642.

Electrical signal 644 is transmitted to orientation transducers 653A and653B. Information contained in signal 644 is used as the error signal ina servo system to alter the orientation of beam steering mirror 652 byorientation transducers 653A and 653B so as to maintain the direction ofpropagation of beam 633 substantially constant independent of changes oforientation of measurement object mirror 662. Orientation transducers653A and 653B changes, in response to signal 644, the orientation ofbeam steering mirror 652 preferably about a point of referenceretroreflector 1532, changes in orientation of beam steering mirror 1522about a nodal point of reference retroreflector 1532 producingsubstantially reduced lateral shear effects experienced by the referencebeam. As a result, the exit reference and Under the condition that thephysical path length of the measurement beam from the first surface ofbeam steering mirror 652 to measurement object mirror 662 is equal tothe physical path length of the measurement beam from measurement objectmirror 662 to the second surface of beam steering mirror 652, there willbe no lateral shear of beam 633 at beam steering mirror 652 when changesin orientation of measurement object mirror 662 are compensated bychanges in orientation of beam steering mirror 652. This condition withrespect to physical path lengths will be referenced as the sixthembodiment condition.

Under the sixth embodiment condition, projections of measurement beams611 and 633 at the surface of beam steering mirror 652 onto a planeparallel to and in the center of the surface planes of beam steeringmirror 652 can be maintained superimposed, changes in orientation ofbeam steering mirror 652 by orientation transducers 653A and 653B inresponse to signal 644 being about the center of the projections ofmeasurement beams 611 and 633 at the surface of beam steering mirror 652onto the plane parallel to and in the center of the surface planes ofbeam steering mirror 652.

The remaining description of the sixth embodiment is the same ascorresponding portions given for the first embodiment.

The six embodiment of the present invention exhibits advantagesadditional to those described for the first embodiment of the presentinvention. The additional advantages include no changes in any part ofthe path of the reference beam associated with changes in orientation ofthe beam steering mirror 652 and/or the measurement object mirror 662and no changes in the path of the measurement beam 611, measurement beam633, and measurement beams and beam components for which beam 633 is aprogenitor that are associated with changes in orientation of thesteering mirror 652 and/or the measurement object mirror 662. Further,under the sixth embodiment condition, the size of the beam steeringmirror 652 need only be large enough to accommodate the projected sizeof measurement beam 611, the location of measurement beams 611 and 633being stationary at beam steering mirror 652. This leads to anadditional advantage for the sixth embodiment with respect to a reducedminimum size permitted for beam steering mirror 652 and orientationtransducers 653A and 653B and concomitant improvement in frequencyresponse of beam steering mirror assembly.

FIGS. 7a and 7 b depict, in schematic form, the seventh embodiment ofthe present invention for measuring a linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Theseventh embodiment is from the second class of embodiments and variantsoperating in the first mode of operation for changes in orientation ofthe object about a first axis and in the second mode of operation forchanges in orientation of the object about a second axis orthogonal tothe first axis and/or a lateral displacement of the object, the lateraldisplacement being orthogonal to both the second axis and the lineardisplacement.

The interferometer system of the seventh embodiment is the same as theinterferometer system of the sixth embodiment except with respect to thereflecting properties of the object: the reflecting properties of theobject for the sixth embodiment are those of a plane mirror and thereflecting properties of the object for the seventh embodiment are thoseof a roof prism. There are also important similarities between the thirdembodiment and the seventh embodiment, the reflecting properties ofobject for both the third and seventh embodiments being those of a roofprism.

In the seventh embodiment, the separation of the measurement beamcomponents incident on and reflected from the object is accomplished byspatially separating the measurement beam components incident on andreflected from the object with a roof prism furnishing the reflectionfunction of the object, the same as for the third embodiment. In thesixth embodiment, the separation of the measurement beam componentsincident on and reflected from the object is accomplished by use ofpolarization encoding, a quarter wave phase retardation plate, and apolarizing beam splitter.

Many elements of the sixth and seventh embodiments perform likefunctions and these elements are indicated by a numeric numbering schemewherein the numeric number of elements performing like functions for theseventh embodiment are equal to the numeric number of correspondingelements of the sixth embodiment incremented by 100. The description ofcertain sections of the reference and measurement beam paths for theseventh embodiment are the same as the description given forcorresponding paths of the sixth embodiment. The certain sections areindicated for the seventh embodiment by a numeric numbering schemewherein the numeric number of the certain section of the reference ormeasurement path of the seventh embodiment having the same descriptionas a corresponding path section of the sixth embodiment is equal to thenumeric number of corresponding path section of the sixth embodimentincremented by 100.

The descriptions of the generation of measurement beam 713 and referencebeam 716 are the same as the descriptions given for correspondingdescriptions of the generation of measurement beam 613 and referencebeam 616 of the sixth embodiment. As shown in FIG. 7a, beam 613 isreflected by mirrors 754A and 754B to form beam 701. Mirrors 754A and754B together produce a constant deviation between beams 713 and 701 inthe plane of FIG. 6a, the same as a penta prism. Beam 701 is reflectedby measurement object roof prism 762 as beam 703 polarized in the planeof FIG. 7a. Beam 703 enters image inverter 756 and exits as beam 705.Image inverter 756 inverters an image about one axis and the descriptionof image inverter 756 is the same as the description of image inverter656 of the sixth embodiment. The primary function of image inverter 756in the seventh embodiment is the same as the primary function of imageinverter 656 in the sixth embodiment.

Reference beam 712 enters retroreflector 770 and is retroreflected as718 polarized orthogonal to the plane of FIG. 7a. The description of thegeneration of beams 738 and 742 from beams 705 and 718 in the seventhembodiment is the same as the corresponding description given for thegeneration of beams 638 and 642 from beams 631 and 616 in the sixthembodiment.

The description of the generation of electronic signal 744 from thedetection of beam 738 in the seventh embodiment is the same as thedescription given in the third embodiment for the generation ofelectronic signal 340 from beam 332. The description of the generationof electronic signal 746 from the detection of beam 746 in the seventhembodiment is the same as the description given in the third embodimentfor the generation of electronic signal 342 from beam 336.

The remaining description of the seventh embodiment is the same ascorresponding portions of descriptions given for the third and sixthembodiments.

The advantage of the seventh embodiment in relation to the sixthembodiment is the same advantage the third embodiment has with respectto the first embodiment.

A first variant of the seventh embodiment of the present invention isdescribed for measuring the linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Thefirst variant of the seventh embodiment is from the second class ofembodiments and variants thereof operating in the second mode ofoperation for changes in orientation of the object about two orthogonalaxes and/or for a lateral displacement of the object, the lateraldisplacement being orthogonal to the linear displacement.

The interferometer system and at least one beam steering assembly of thefirst variant of the seventh embodiment comprise the same elements asthe interferometer system and at least one beam steering assembly of theseventh embodiment except for the object element, the detector of acertain optical beam, signal produced by the detector of the certainoptical beam, and respective servo system.

The measurement object element of the seventh embodiment, roof prism762, is replaced by a measurement retroreflector 762A (not shown in afigure) in the first variant of the seventh embodiment. As aconsequence, the relative directions of propagation of the reference andmeasurement beam components of output beam 738A, the output beam of thefirst variant of the seventh embodiment corresponding to output beam 738of the seventh embodiment, is substantially independent of changes inorientation of the measurement object retroreflector 762A of the firstvariant of the seventh embodiment. However a lateral translation ofmeasurement object retroreflector 762A in a plane orthogonal to thedirection of the linear displacement will generate a lateraldisplacement of measurement beam component of output beam 738A.

Detector 776A of the first variant of the seventh embodiment,corresponding to detector 776 of the seventh embodiment, detects thelateral displacement or shear of the measurement beam component ofoutput beam 738A and generates electrical signal 744A, the electricalsignal corresponding to electrical signal 744 of the seventh embodiment.

Electrical signal 744A is transmitted to orientation transducers 753Aand 753B of the first variant of the seventh embodiment. Informationcontained in signal 744A is used as the error signal in a servo systemto alter the orientation of beam steering mirror 752 by orientationtransducers 753A and 753B of the first variant of the seventh embodimentso as to maintain the lateral location of measurement beam component ofoutput beam 738A substantially constant independent of a lateraltranslation of measurement object retroreflector 762A in a planeorthogonal to the direction of the linear displacement and/orsubstantially independent of changes in orientation of measurementobject retroreflector 762A about two orthogonal axes which are nominallyorthogonal to the direction of propagation of beam 701 of the firstvariant of the seventh embodiment.

The remaining description of the first variant of the seventh embodimentis the same as corresponding portions of the description given for theseventh embodiment.

FIGS. 8a-8 c depict in schematic form an eighth embodiment of thepresent invention for measuring a linear displacement of an object witha fixed orientation wherein the direction of propagation of themeasurement beam reflected from the object is sensitive to changes inorientation of the object and a high transmission efficiency ismaintained for the measurement beam without the use of polarizationencoding. The eighth embodiment is from the second class of embodimentsand variants operating in the second mode of operation for changes inthe linear displacement of the object.

The description of light beam 810 shown in FIG. 8a and of a source oflight beam 810 is the same as the description of light beam 110 and of asource of light beam 110 given for the first embodiment. As shown inFIG. 8a, beam 810 impinges on polarizing beam splitter 850. A portion ofbeam 810 is transmitted as measurement beam 811 polarized in the planeof FIG. 8a and a second portion of beam 810 is reflected as referencebeam 812 orthogonally polarized to the plane of FIG. 8a.

In FIG. 8a, beam 811 is reflected by a first surface of mirror 852 asbeam 813. Beam 813 enters image inverter 854. As shown in FIG. 8b, imageinverter 854 is comprised of three mirrors 854A, 854B, and 854C. Beam813 is reflected by each of the three mirrors 854A, 854B, and 854C andexits mirror assembly 854 as beam 817. Image inverter 854 inverters animage about one axis and the description of image inverter 854 is thesame as the description of image inverter 656 of the sixth embodiment.

Beam 817 is reflected by beam steering mirror 856 as beam 819. Beamsteering mirror 856 and orientation transducers 857A and 857B comprise abeam steering assembly that redirects beam 819. Beam 819 is reflected bymeasurement object mirror 858 as beam 823. Beam 823 is reflected bymirror 860 as beam 825 and beam 825 is reflected by beam steering mirror862 as beam 827. Beam steering mirror 862 and orientation transducers863A and 863B comprise a beam steering assembly that redirects beam 827.

Beam 827 is reflected by mirrors 864A, 864B, and 864C to form beam 829.Mirrors 864A, 864B, and 864C together comprise an image inverter whichinverters an image about one axis orthogonal to plane of FIG. 8a and thedescription of which is the same as the description of image inverter656 of the sixth embodiment. The primary function of the image invertercomprised of mirrors 864A, 864B, and 864C is the same as the primaryfunction of image inverter 656 in the sixth embodiment. The angle ofincidence of beam 829 on mirror 852 is nominally the same as the angleof incidence of beam 811 on mirror 852.

Beam 829 is reflected from a second surface of mirror 852 as beam 833. Atranslation of measurement object mirror 858 with a fixed orientation ina direction parallel to the average directions of propagation of beams819 and 823 will effect a change in the lateral location of beam 833 atmirror 852. It is possible to compensate for the effect on beam 833 byappropriate changes in the orientations of beam steering mirrors 856 and862, the appropriate changes being accomplished by the respectiveorientation transducers in conjunction with the combination of mirror860 and measurement object mirror 858 located in the path of themeasurement beam between beam steering mirrors 856 and 862.

Beam 833 is reflected by mirror 866 as beam 835. Beam 835 is polarizedin the plane of FIG. 8a.

Reference beam 812 enters image inverter 868. As shown in FIG. 8c, imageinverter 868 is comprised of three mirrors 868A, 868B, and 868C. Theplane of FIG. 8c is orthogonal to the plane of FIG. 8a. Beam 812 isreflected by each of the three mirrors 868A, 868B, and 868C and exitsmirror assembly 868 as beam 816. The primary function of the imageinverter 868 is the same as the primary function of image inverter 656in the sixth embodiment.

Reference beam 816 is subsequently reflected by mirrors 870A and 870B toform beam 818. Mirrors 870A and 870B function together as a roof prism.Beam 818 is polarized perpendicular to the plane of FIG. 8a.

Beam 835 is transmitted by polarizing beam splitter 872 as one componentof output beam 836, beam 835 being polarized in the plane of FIG. 8a. Aportion of beam 816 is transmitted by polarizing beam splitter 872 as asecond component of output beam 836, beam 816 being polarized orthogonalto the plane of FIG. 8a.

A portion of output beam 836 is reflected by nonpolarizing beam splitter874 as beam 838. Beam 838 impinges on detector 876 to produce anelectrical signal 845 related to the lateral location of the measurementbeam component in beam 838.

A second portion of output beam 836 is transmitted by nonpolarizing beamsplitter 874 as beam 840. Beam 840 is transmitted by a polarizer 878orientated to produce a mixed optical beam 842. Beam 842 impinges ondetector 880 to generate electrical signal 846 related to the differencein phase of the measurement and reference beam components of mixed beam842. Electrical signal 846 is transmitted to electronic processor andcomputer 882 for subsequent processing for the information about thedifference in phase of the measurement and reference beam components ofmixed beam 842.

The electronic signal 845 is used by orientation transducers 857A and857B to effect a rotation of steering mirror 856 in the plane of FIG. 8aabout point R₁ shown in FIG. 8a. Similarly, electronic signal 845 isused by orientation transducers 863A and 863B to effect a rotation ofsteering mirror 862 about point R₂ in the plane of FIG. 8a. For eachrotation of beam steering mirror 856 produced by orientation transducers857A and 857B resulting from a given signal 845, the same angularrotation of beam steering mirror 862 is generated by orientationtransducers 863A and 863B also based on signal 845. Thus the directionof propagation of beam 827 is unaltered by the combined rotations ofbeam steering mirrors 856 and 862 arising from a given signal 845.

The line of length y₁ from R₁ to the point where beam 817 intersectsbeam steering mirror 856 is orthogonal to the direction of propagationof beam 817. The line of length y₂ from R₂ to the point where beam 827intersects beam steering mirror 862 is orthogonal to the direction ofpropagation of beam 827. Under a set of eighth embodiment conditionscomprising the condition that:

y₁=y₂,   (2)

the condition that the physical path length of beam 819 is equal to thecombined physical path lengths of beams 823 and 825, and the conditionthat points R₁ and R₂ are coextensive, beam steering mirrors 856 and 862can each be rotated by an angle γ in the plane of FIG. 8a about pointsR₁ and R₂, respectively, such that for a translation of measurementobject mirror 858 of fixed orientation in a direction parallel to theaverage direction of propagation of beams 819 and 823 before thetranslation,

1) there will be no change in the average direction of propagation ofbeams 819 and 823;

2) there will be no lateral shear introduced in beam 819 at measurementobject mirror 858;

3) there will be no lateral shear introduced in beam 817 at beamsteering mirror 856 or in beams which are progenitors of beam 817; and

4) there will be no lateral shear introduced in beam 827 at beamsteering mirror 862 or in measurement beams or measurement beamcomponents for which beam 827 is a progenitor.

The absolute magnitude of the changes in angles of rotation γ will beequal to the absolute magnitude of changes in η/2 where η is the angleof incidence of beam 819 at measurement object mirror 858. The magnitudeof the changes in angles of rotation γ encountered in typical operationof a distance measuring interferometer will be of the order of 0.005radians for measurement leg lengths varying between 0.4 m and 1.0 m.

The remaining description of the eight embodiment is the same ascorresponding portions of the descriptions given for the sixthembodiment.

An advantage of the eighth embodiment is a high transmission efficiencyof the measurement beam without the use of polarization encoding andwithout the introduction of variable lateral displacements ofmeasurement beam 811, measurement beam 833, and measurement beams andmeasurement beam components for which beam 833 is a progenitor, theadvantage being present for a measurement of a linear displacement of anobject of a fixed orientation wherein the direction of propagation ofthe measurement beam reflected from the object is sensitive to changesin orientation of the object.

FIGS. 9a-9 c depicts in schematic form a ninth embodiment of the presentinvention for measuring a linear displacement of an object substantiallyindependent of the orientation of the object and substantiallyindependent of a lateral displacement of the object, the lateraldisplacement being orthogonal to the linear displacement. The ninthembodiment is from the second class of embodiments and variantsoperating in the first mode of operation for changes in orientation ofthe object about two orthogonal axes, the orthogonal axes beingorthogonal to the linear displacement and/or a lateral displacement ofthe object, the lateral displacement being orthogonal to the lineardisplacement.

The ninth embodiment incorporates the beam steering beam assemblies ofthe sixth and eighth embodiments. In this role, the elements of theninth embodiment shown in FIG. 9a perform like functions as likenumbered elements of the eighth embodiment shown in FIG. 8a incrementedby 100 except for beam steering mirror 952 and orientation transducers953A and 953B. The description of beam steering mirror 952 andorientation transducers 953A and 953B is the same as the descriptiongiven for beam steering mirror 652 and orientation transducers 653A and653B of the sixth embodiment.

Detector 976 performs the functions of detector 676 of the sixthembodiment and generates electronic signal 944, the description ofelectronic signal 944 of the ninth embodiment being the same ascorresponding portions of the description given for electronic signal644 of the sixth embodiment. Detector 976 in addition performs thefunctions of detector 876 of the eighth embodiment and generateselectronic signal 945, the description of electronic signal 945 beingthe same as the description given for electronic signal 845 of theeighth embodiment.

In the normal mode operation of the ninth embodiment, if the orientationof beam steering mirror 952 is not correct for a given orientation ofmeasurement object mirror 958, there will be a difference in thedirections of propagation of the measurement beam and reference beamcomponents of beam 938 and a lateral shear of the measurement beamcomponent relative to the reference beam component of beam 938. Detector976 detects the difference in directions of propagation of themeasurement and reference beam components of beam 938 and generateselectronic signal 944. Electronic signal 944 is transmitted toorientation transducers 953A and 953B which adjust the orientation ofbeam steering mirror 952 so that the directions of propagation of themeasurement and reference beam components of beam 938 are the same. Thelateral shear of the measurement beam component relative to thereference beam component of beam 938 due to the incorrect orientation ofbeam steering mirror 952 is compensated simultaneously with thecompensation of the difference in the directions of propagation of themeasurement beam and reference beam components of beam 938.

There will be a residual lateral shear of the measurement beam componentrelative to the reference beam component of beam 938 should there be anincorrect orientation of beam steering mirrors 956 and 962. Detector 976detects the residual lateral shear of the measurement and reference beamcomponents of beam 938 and generates electronic signal 945. Electronicsignal 945 is transmitted to orientation transducers 957A and 957B whichadjust the orientation of beam steering mirror 956 and to orientationtransducers 963A and 963B which adjust the orientation of beam steeringmirror 962 so that the residual lateral shear of the measurement andreference beam components of beam 938 is compensated.

The remaining description of the ninth embodiment is the same as thatgiven for corresponding portions of the descriptions given for the sixthand eight embodiments.

FIGS. 10a-10 c depict in schematic form a tenth embodiment of thepresent invention for measuring a linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement, andwherein a high transmission efficiency of the measurement beam ismaintained without the use of polarization encoding. The tenthembodiment is from the second class of embodiments and variantsoperating in the first mode of operation for changes in orientation ofthe object about two orthogonal axes, two orthogonal axes beingorthogonal to the linear displacement and/or a lateral displacement ofthe object, the lateral displacement being orthogonal to the lineardisplacement.

The tenth embodiment may be considered a variant of the ninth embodimentof the present invention. The function performed by mirror assemblycomprised of beam steering mirror 1052 and orientation transducers 1053Aand 1053B of the tenth embodiment is the same function performed by themirror assembly comprised of mirror 952 and orientation transducers 953Aand 953B of the ninth embodiment. The function performed by mirrorassembly comprised of beam steering mirror 1058 and orientationtransducers 1059A and 1059B of the tenth embodiment is the same as thecombined functions performed by the mirror assembly comprised of beamsteering mirror 956 and orientation transducers 957A and 957B and beamsteering mirror assembly comprised of mirror 962 and orientationtransducers 963A and 963B of the ninth embodiment.

The description of electronic signals 1026 and 1028 of the tenthembodiment is the same as the corresponding portions of the descriptionsgiven for electronic signals 944 and 945, respectively, of the ninthembodiment. The description of the remaining elements of the tenthembodiment shown in FIG. 10a perform like functions as like numberedelements of the ninth embodiment shown in FIG. 9a incremented by 100.

The relevant tenth embodiment conditions are that:

1) the physical path length from measurement beam 1013 at beam steeringmirror 1058 to measurement beam 1021 at beam steering mirror 1052 isequal to the physical path length from measurement beam 1039 at beamsteering mirror 1052 to measurement beam 1041 at steering mirror 1058;

2) the physical path length from measurement beam 1023 at beam steeringmirror 1052 to measurement beam 1031 at measurement object mirror 1064is equal to the physical path length from measurement beam 1033 atmeasurement object mirror 1064 to measurement beam 1037 at beam steeringmirror 1052;

3) the angle of incidence of beam 1011 and the angle of incidence ofbeam 1041 at beam steering mirror 1058 are nominally the same; and

4) the angle of incidence of beam 1021 and the angle of incidence ofbeam 1037 at beam steering mirror 1052 are nominally the same.

It will be evident to those skilled in the art that under the tenthembodiment conditions that the location of beam 1011 and the location ofbeam 1043 at beam steering mirror 1058 will be substantially one abovethe other and that the location of beam 1021 and the location of beam1039 at beam steering mirror 1052 will be substantially one above theother.

The description of the steering beam assembly comprising steering mirror652 of the sixth embodiment with respect to the size of respectivecomponents and frequency response is applicable for both of the beamsteering assemblies of the tenth embodiment comprising beam steeringmirrors 1052 and 1058.

The remaining description of the tenth embodiment is the same ascorresponding portions of the description given for the ninthembodiment.

FIGS. 11a-11 f depict, in schematic form, the eleventh embodiment of thepresent invention for measuring a linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Theeleventh embodiment is from the second class of embodiments and variantsthereof operating in the first mode of operation for changes inorientation of the object about two orthogonal axes, two orthogonal axesbeing orthogonal to the linear displacement, and/or a lateraldisplacement of the object, the lateral displacement being orthogonal toboth the linear displacement.

The interferometer system of the tenth and eleventh embodiments aregenerically similar except with respect to the number of passes themeasurement beam makes to the object: the measurement beams of theeleventh embodiment traverse certain sections of the measurement path intwo opposite directions of propagation where in the tenth embodiment,the corresponding measurement beam transits the certain sections in onlyone direction of propagation. Efficient separation of an outputmeasurement beam component from an input measurement beam component inthe multiple pass configuration of the eleventh embodiment is effectedby use of polarization encoding and polarizing beam splitters.

Many elements of the tenth and eleventh embodiments perform likefunctions and these elements are indicated by a numeric numbering schemewherein the numeric number of elements performing like functions for theeleventh embodiment are equal to the numeric number of correspondingelements of the tenth embodiment incremented by 100. The certainsections of the reference and measurement beam paths are indicated bythe numeric numbering scheme wherein the numeric number of the certainsection of the reference or measurement path of the eleventh embodimenthaving the same description as a corresponding path section of the tenthembodiment is equal to the numeric number of corresponding path sectionof the tenth embodiment incremented by 100.

For the certain sections of the measurement beam paths for the eleventhembodiment, there are measurement beams traversing the certain sectionsin a reverse direction of propagation, the nonreverse or forwarddirection of propagation being associated with a first pass tomeasurement object mirror 1164 and the reverse direction of propagationbeing associated with a second pass to the measurement object mirror1164. The description of the additional measurement beams traversing thecertain sections in a reverse direction are otherwise the same as thecorresponding portions of the description given associated measurementbeams traversing in a nonreverse direction of propagation for the tenthembodiment, an alphanumeric number of a measurement beam traversing in areverse direction being equal to the numeric number of the associatedmeasurement beam traversing in a nonreverse direction augmented with thesuffix R.

The descriptions of the generation of measurement beam 1143 andreference beam 1112 is the same as the descriptions given forcorresponding descriptions of the generation of measurement beam 1043and reference beam 1012 of the tenth embodiment. As shown in FIG. 11d, afirst portion of beam 1143 is transmitted by nonpolarizing beam splitter1171 as beam 1145. Beam 1145 impinges on detector 1190 to generateelectrical signal 1128. A second portion of beam 1143, being polarizedin the plane of FIG. 11d, is reflected by nonpolarizing beam splitter1171, is transmitted by quarter wave phase retardation plate 1172 as acircularly polarized beam, reflected by mirror 1173 as a beam withopposite-handed circular polarization, transmitted by phase retardationplate 1172 as a beam orthogonally polarized to the plane of FIG. 11d,and subsequently a portion thereof reflected by nonpolarizing beamsplitter 1171 as beam 1143R. Beam 1143R is orthogonally polarized to theplane of FIG. 11d.

The elements of detector 1190 are depicted, in schematic form, in FIG.11e. Beam 1145 enters detector 1190 and a first portion of beam 1145 isreflected by nonpolarizing beam splitter 1190A as beam 1145A. Beam 1145Ais focused by lens 1190B to a spot on detector 1190C such as a quad celldetector, a two-dimensional high speed CCD camera, or a lateral effectphotodiode. Two-dimensional data arrays are transmitted as electronicsignal 1128A. The location of the spot on detector 1190C is not effectedby a lateral shear of beam 1145A but a change in the direction ofpropagation of beam 1145A does shift the location of the spot ondetector 1190C. Thus the electronic signal 1128A contains informationabout the direction of propagation of beam 1145A.

A second portion of beam 1145 is transmitted as beam 1145B. Beam 1145Bimpinges on detector 1190D such as a quad cell to generate electronicsignal 1128B. The location of beam 1145B on detector 1190D is effectedby a lateral shear of beam 1145B but a change in the direction ofpropagation of beam 1145B at detector 1190D does shift the location ofbeam 1145B on detector 1190D. Thus the electronic signal 1128B containsinformation about lateral shear of beam 1145B. Electronic signals 1128Aand 1128B comprise electronic signal 1128.

As shown in FIG. 11a, a first portion of reference 1112 is transmittedby nonpolarizing beam splitter 1174 as beam 1114 and a second portion ofreference 1112 is reflected by nonpolarizing beam splitter 1174 as beam1116. As shown in FIGS. 11d and 11 f, beam 1114 impinges on detector1192 to generate electrical signal 1130.

The elements of detector 1192 are depicted, in schematic form, in FIG.11f. Beam 1114 enters detector 1192 and a first portion of beam 1114 isreflected by nonpolarizing beam splitter 1192A as beam 1114A. Beam 1114Ais focused by lens 1192B to a spot on detector 1192C such as a quad celldetector, a two-dimensional high speed CCD camera, or a lateral effectphotodiode. Two-dimensional data arrays are transmitted as electronicsignal 1130A. The location of the spot on detector 1192C is not effectedby a lateral shear of beam 1114A but a change in the direction of beam1114A does shift the location of the spot on detector 1192C. Thus theelectronic signal 1130A contains information about the direction ofpropagation of beam 1114A.

A second portion of beam 1114 is transmitted by nonpolarizing interface1192A as beam 1114B. Beam 1114B impinges on detector 1192D such as aquad cell to generate electronic signal 1130B. The location of beam1114B on detector 1192D is effected by a lateral shear of beam 1114B buta change in the direction of propagation of beam 1114B at detector 1192Ddoes shift the location of beam 1114B on detector 1192D. Thus theelectronic signal 1130B contains information about lateral shear of beam1114B. Electronic signals 1130A and 1130B comprise electronic signal1130.

Electronic signals 1128 and 1130 are transmitted to electronic signalprocessor 1194 (cf. FIG. 11d). From the components of electronic signals1128 and 1130 that contain information about the directions ofpropagation of measurement beam 1145A and reference beam 1114A,electronic processor 1194 generates electronic signal 1132. From thecomponents of electronic signals 1128 and 1130 that contain informationabout the lateral shear of measurement beam 1145B and reference beam1114B, electronic processor 1194 generates electronic signal 1134.

Electrical signal 1132 is transmitted to orientation transducers 1153Aand 1153B and electronic signal 1134 is transmitted to orientationtransducers 1159A and 1159B. Information contained in signal 1132 isused as the error signal in a servo system to alter the orientation ofbeam steering mirror 1152 by orientation transducers 1153A and 1153B soas to maintain in two orthogonal dimensions the direction of propagationof measurement beam 1143 substantially constant independent of changesof orientation of measurement object mirror 1164. Information containedin signal 1134 is used as the error signal in a servo system to alterthe orientation of beam steering mirror 1158 by orientation transducers1159A and 1159B so as to maintain in two orthogonal dimensions thelateral position of measurement beam 1143 substantially constantindependent of changes in the location of measurement object mirror 1164along the average of the paths of beams 1131 and 1133. Under a set ofeleventh embodiment conditions that the physical path length of themeasurement beam from the location of the first redirection of themeasurement beam by the beam steering element 1158 to the firstredirection of the measurement beam by the beam steering element 1152 isequal to the physical path length of the measurement beam from thesecond redirection of the measurement beam by the beam steering element1152 to the second redirection of the measurement beam by the beamsteering element 1158 and that the physical path length of themeasurement beam from the location of the first redirection of themeasurement beam by the beam steering element 1152 to measurement objectmirror 1164 is equal to the physical path length of the measurement beamfrom measurement object mirror 1164 to the location of the secondredirection of the measurement beam by the beam steering mirror 1152,there will be no lateral shear of beam 1143 at beam steering element1158 when changes in orientation of measurement object mirror 1164 arecompensated by changes in orientation of beam steering element 1152 andwhen the effects of changes in the location of measurement object mirror1164 along the average of the paths of beams 1131 and 1133 arecompensated by changes in orientation of beam steering mirror 1158.These conditions with respect to physical path lengths will bereferenced as the eleventh embodiment conditions.

Beam 1143R is substantially coextensive with beam 1143 and beam 1143R(cf. FIG. 11a) has a direction of propagation substantially opposite tothe direction of propagation of 1143. As a consequence, beam 1143Rsubstantially retraces the pass through portions of the interferometer,the pass leading to the generation of beam 1143, to generate beam 1111R.The retrace through portions of the interferometer includes inparticular a pass to the measurement object mirror 1164. Beam 1111R issubstantially coextensive with beam 1111 and beam 1111R has a directionof propagation substantially opposite to the direction of propagation ofbeam 1111.

Beam 1111R, being polarized orthogonal to the plane of FIG. 11a, isreflected by polarizing beam splitter 1150 as beam 1147. Beam 1147 isreflected by mirror 1177 as beam 1149.

Reference beam 1116 is reflected by mirrors 1176A and 1176B to form beam1118. Mirrors 1176A and 1176B together produce a constant deviationbetween beams 1116 and 1118 in the plane of FIG. 11a, the same as apenta prism.

A portion of beam 1149 is transmitted by nonpolarizing beam splitter1178 as one component of output beam 1124. A portion of beam 1118 istransmitted by nonpolarizing beam splitter 1178 as a second component ofthe output beam 1124. Beam 1124 is a mixed optical beam, beams 1149 and1118 being both polarized orthogonal to the plane of FIG. 11a and beamsplitter 1178 being of the nonpolarizing type. Beam 1124 impinges ondetector 1186 to generate electrical signal 1136 related to thedifference in phase of the measurement and reference beam components ofmixed beam 1124. Electrical signal 1136 is transmitted to electronicprocessor and computer 1188 for subsequent processing to obtaininformation about the difference in phase of the measurement andreference beam components of mixed beam 1124. The detection of mixedbeam is typically by photoelectric detection to produce a heterodynesignal, the phase of which is related to the difference in optical pathlengths of the measurement and reference beams. The phase of theheterodyne signal may be determined by electronic processor and computer1188 for example from a Fourier transform or Hilbert transform of theheterodyne signal.

The remaining description of the eleventh embodiment is the same ascorresponding portions given for the descriptions of the fourth andtenth embodiments.

FIGS. 12a-12 e depict, in schematic form, the twelfth embodiment of thepresent invention for measuring a linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Thetwelfth embodiment is from the second class of embodiments and variantsthereof operating in the first mode of operation for changes inorientation of the object about a first axis and in the second mode ofoperation for changes in orientation of the object about a second axisorthogonal to the first axis, the first and second axes being orthogonalto the linear displacement, and/or a lateral displacement of the object,the lateral displacement being orthogonal to both the second axis andthe linear displacement.

The interferometer system of the seventh and twelfth embodiments aregenerically similar except with respect to the number of passes themeasurement beam makes to the object: the measurement beams of thetwelfth embodiment traverse certain sections of the measurement path intwo opposite directions of propagation where in the seventh embodiment,the corresponding measurement beam transits the certain sections in onlyone direction of propagation. Efficient separation of an outputmeasurement beam component from an input measurement beam component inthe multiple pass configuration of the twelfth embodiment is effected byuse of polarization encoding and polarizing beam splitters.

Many elements of the seventh and twelfth embodiments perform likefunctions and these elements are indicated by a numeric numbering schemewherein the numeric number of elements performing like functions for thetwelfth embodiment are equal to the numeric number of correspondingelements of the eleventh embodiment incremented by 500. The certainsections of the measurement beam paths are indicated by a numericnumbering scheme wherein the numeric number of the certain section ofthe measurement path of the twelfth embodiment having the samedescription as a corresponding path section of the seventh embodiment isequal to the numeric number of corresponding path section of the seventhembodiment incremented by 500.

For the certain sections of the measurement beam paths for the twelfthembodiment, there are measurement beams traversing the certain sectionsin a reverse direction of propagation, the nonreverse or forwarddirection of propagation being associated with a first pass tomeasurement object roof prism 1262 and the reverse direction ofpropagation being associated with a second pass to the measurementobject roof prism 1262. The description of the additional measurementbeams traversing the certain sections in a reverse direction areotherwise the same as the corresponding portions of the descriptiongiven associated measurement beams traversing in a nonreverse directionof propagation for the seventh embodiment, an alphanumeric number of ameasurement beam traversing in a reverse direction being equal to thenumeric number of the associated measurement beam traversing in anonreverse direction augmented with the suffix R.

The descriptions of the generation of measurement beam 1233 andreference beam 1212 is the same as the descriptions given forcorresponding descriptions of the generation of measurement beam 733 andreference beam 712 of the seventh embodiment. As shown in FIG. 12c, afirst portion of beam 1233 is transmitted by nonpolarizing beam splitter1271 as beam 1235. Beam 1235 impinges on detector 1290 to generateelectrical signal 1228. A second portion of beam 1233, being polarizedin the plane of FIG. 12c, is reflected by nonpolarizing beam splitter1271, is transmitted by quarter wave phase retardation plate 1272 as acircularly polarized beam, reflected by mirror 1273 as a beam withopposite-handed circular polarization, transmitted by quarter wave phaseretardation plate 1272 as a beam orthogonally polarized to the plane ofFIG. 12c, and subsequently a portion thereof reflected by nonpolarizingbeam splitter 1271 as beam 1233R. Beam 1233R is orthogonally polarizedto the plane of FIG. 12c.

The elements of detector 1290 are depicted, in schematic form, in FIG.12d. Beam 1235 enters detector 1290 and a first portion of beam 1235 isreflected by nonpolarizing beam splitter 1290A as beam 1235A. Beam 1235Ais focused by lens 1290B to a spot onto a one dimensional detector 1290Csuch as a dual cell detector, a linear array, high speed CCD camera, ora lateral effect photodiode. An one-dimensional data array istransmitted as electronic signal 1228A. The location of the spot on onedimensional detector 1290C is not effected by a lateral shear of beam1235A but a change in the direction of beam 1235A in the plane definedby the one dimensional detector 1290C and the nominal direction of beam1235A does shift the location of the spot on detector 1290C. Thus theelectronic signal 1228A contains information about the direction ofpropagation of beam 1235A in the plane defined by the one dimensionaldetector 1290C and the nominal direction of beam 1235A, a planecorresponding to a plane orthogonal to the plane of FIG. 12a andorthogonal to the nominal direction of propagation of measurement beam1201.

A second portion of beam 1235 is transmitted as beam 1235B. Beam 1235Bimpinges on one dimensional detector 1290D such as a dual cell detectorto generate electronic signal 1228B. The location of beam 1235B ondetector 1290D is effected by a lateral shear of beam 1235B but a changein the direction of propagation of beam 1235B at detector 1290D doesshift the location of beam 1235B on one dimensional detector 1290D. Thusthe electronic signal 1228B contains information about lateral shear ofbeam 1235B, the plane defined by the one dimensional detector 1290Dbeing orthogonal to the plane defined by the one dimensional detector1290C. Electronic signals 1228A and 1228B comprise electronic signal1228.

As shown in FIG. 12a, a first portion of reference 1212 is transmittedby nonpolarizing beam splitter 1274 as beam 1214 and a second portion ofreference 1212 is reflected by nonpolarizing beam splitter 1274 as beam1216. As shown in FIGS. 12c and 12 e, beam 1214 impinges on detector1292 to generate electrical signal 1230.

The elements of detector 1292 are depicted, in schematic form, in FIG.12e. Beam 1214 enters detector 1292 and a first portion of beam 1214 isreflected by nonpolarizing beam splitter 1292A as beam 1214A. Beam 1214Ais focused by lens 1292B to a spot on a one dimensional detector 1292Csuch as a dual cell detector, linear array high, speed CCD camera, or alateral effect photodiode. A one-dimensional data array is transmittedas electronic signal 1230A. The location of the spot on the onedimensional detector 1292C is not effected by a lateral shear of beam1214A but a change in the direction of propagation of beam 1214A doesshift the location of the spot on the one dimensional detector 1292C.Thus the electronic signal 1230A contains information about thedirection of propagation of beam 1214A in the plane defined by the onedimensional detector 1292C and the nominal direction of beam 1214A theplane of one dimensional detector 1292C arranged to correspond to theplane of one dimensional detector 1290C.

A second portion of beam 1214 is transmitted as beam 1214B. Beam 1214Bimpinges on a one dimensional detector 1292D such as a dual celldetector to generate electronic signal 1230B. The location of beam 1214Bon detector 1292D is effected by a lateral shear of beam 1214B but achange in the direction of propagation of beam 1214B at detector 1292Ddoes shift the location of beam 1214B on detector 1292D. Thus theelectronic signal 1230B contains information about lateral shear of beam1214B, the plane defined by the one dimensional detector 1292D arrangedto correspond. to the plane defined by the one dimensional detector1290D. Electronic signals 1230A and 1230B comprise electronic signal1230.

Electronic signals 1228 and 1230 are transmitted to electronic signalprocessor 1294 (cf. FIG. 12c). From the components of electronic signals1228 and 1230 that contain information about the directions ofpropagation of measurement beam 1235A and reference beam 1214A in aplane orthogonal to the plane of FIG. 12c and parallel to the nominaldirection of propagation of beam 1235A, electronic processor 1294generates electronic signal 1232. From the components of electronicsignals 1228 and 1230 that contain information about the lateral shearof measurement beam 1235B and reference beam 1214B in the plane of FIG.12c, electronic processor 1294 generates electronic signal 1234.

Electrical signal 1232 is transmitted to orientation transducers 1253Aand 1253B. Information contained in signal 1232 is used as the errorsignal in a servo system to alter the orientation of beam steeringmirror 1252 by orientation transducers 1253A and 1253B so as to maintainin a first plane the direction of propagation of measurement beam 1233substantially constant independent of changes of orientation ofmeasurement object roof prism 1262, the first plane being orthogonal tothe plane of FIG. 12a and parallel to the nominal direction ofpropagation of beam 1201, and to maintain in a plane orthogonal to thefirst plane the lateral position of measurement beam 1233 substantiallyconstant independent of translations of measurement object roof prism1262 nominally orthogonal to the direction of propagation of measurementbeam 1201 and substantially independent of changes in the orientation ofmeasurement object roof prism 1262 about a axis orthogonal to the planeof FIG. 12a and orthogonal to the nominal direction of propagation ofbeam 1201. Under the condition that the physical path length of themeasurement beam from the location of the first redirection of themeasurement beam by the beam steering element 1252 to measurement objectroof prism 1262 is equal to the physical path length of the measurementbeam from measurement object roof prism 1262 to the location of thesecond redirection of the measurement beam by the beam steering mirror1252, there will be no lateral shear of beam 1233 at beam steeringelement 1252 when changes in orientation of measurement object roofprism 1262 are compensated by changes in orientation of beam steeringelement 1252 and when the effects of displacements in the location ofmeasurement object roof prism 1262 nominally orthogonal to the path ofbeam 1233 are compensated by changes in orientation of beam steeringelement 1252. This condition with respect to physical path lengths willbe referenced as the twelfth embodiment condition.

Beam 1233R is substantially coextensive with beam 1233 and beam 1233R(cf. FIG. 12c) has a direction of propagation substantially opposite tothe direction of propagation of 1233. As a consequence, beam 1233Rsubstantially retraces the pass through portions of the interferometer,the pass leading to the generation of beam 1233, to generate beam 1211R.The retrace through portions of the interferometer includes inparticular a pass to the object roof prism 1262. Beam 1211R issubstantially coextensive with beam 1211 and beam 1211R has a directionof propagation substantially opposite to the direction of propagation ofbeam 1211.

Beam 1211R, being polarized orthogonal to the plane of FIG. 12a, isreflected by polarizing beam splitter 1250 as beam 1247. Beam 1247 isreflected by mirror 1177 as beam 1249.

Reference beam 1216 is reflected by mirrors 1276A and 1276B to form beam1218. Mirrors 1276A and 1276B together produce a constant deviationbetween beams 1216 and 1218 in the plane of FIG. 12a, the same as apenta prism.

A portion of beam 1249 is transmitted by nonpolarizing beam splitter1278 as one component of output beam 1224. A portion of beam 1218 istransmitted by nonpolarizing beam splitter 1278 as a second component ofthe output beam 1224. Beam 1224 is a mixed optical beam, beams 1249 and1218 being both polarized orthogonal to the plane of FIG. 12a and beamsplitter 1278 being of the nonpolarizing type. Beam 1224 impinges ondetector 1286 to generate electrical signal 1236 related to thedifference in phase of the measurement and reference beam components ofmixed beam 1224. Electrical signal 1236 is transmitted to electronicprocessor and computer 1288 for subsequent processing to obtaininformation about the difference in phase of the measurement andreference beam components of mixed beam 1224. The detection of mixedbeam is typically by photoelectric detection to produce a heterodynesignal, the phase of which is related to the difference in optical pathlengths of the measurement and reference beams. The phase of theheterodyne signal may be determined by electronic processor and computer1288 for example from a Fourier transform or Hilbert transform of theheterodyne signal.

The remaining description of the twelfth embodiment is the same ascorresponding portions given for the descriptions of the fourth andeleventh embodiments.

A first variant of the twelfth embodiment of the present invention isdescribed for measuring the linear displacement of an objectsubstantially independent of the orientation of the object andsubstantially independent of a lateral displacement of the object, thelateral displacement being orthogonal to the linear displacement. Thefirst variant of the twelfth embodiment is from the second class ofembodiments and variants thereof operating in the second mode ofoperation for changes in orientation of the object about two orthogonalaxes and/or for a lateral displacement of the object, the lateraldisplacement being orthogonal to the linear displacement.

The interferometer system and at least one beam steering assembly of thefirst variant of the twelfth embodiment comprise the same elements asthe interferometer system and at least one beam steering element of thetwelfth embodiment except for the object element, the detector of acertain optical beam, signal produced by the detector of the certainoptical beam, and respective servo system.

The object element of the twelfth embodiment, measurement object roofprism 1262, is replaced by a measurement object retroreflector 1262A(not shown in a figure) in the first variant of the twelfth embodiment.As a consequence, the relative directions of propagation of thereference and measurement beam components of output beam 1238A, theoutput beam of the first variant of the twelfth embodiment correspondingto output beam 738 of the seventh embodiment, is substantiallyindependent of changes in orientation of the measurement objectretroreflector 1262A of the first variant of the twelfth embodiment.However a lateral translation of measurement object retroreflector 1262Ain a plane orthogonal to the direction of the linear displacement willgenerate a lateral displacement of measurement beam component of outputbeam 1238A.

Detector 1276A of the first variant of the twelfth embodiment,corresponding to detector 776 of the seventh embodiment, detects thelateral displacement or shear of the measurement beam component ofoutput beam 1238A and generates electrical signal 1244A, the electricalsignal corresponding to electrical signal 744 of the seventh embodiment.

Electrical signal 1244A is transmitted to orientation transducers 1253Aand 1253B of the first variant of the twelfth embodiment. Informationcontained in signal 1244A is used as the error signal in a servo systemto alter the orientation of beam steering mirror 1252 by orientationtransducers 1253A and 1253B of the first variant of the twelfthembodiment so as to maintain the lateral location of measurement beamcomponent of output beam 1238A substantially constant independent of alateral translation of measurement object retroreflector 1262A in aplane orthogonal to the direction of the linear displacement and/orsubstantially independent of changes in orientation of measurementobject retroreflector 1262A about two orthogonal axes which arenominally orthogonal to the direction of propagation of beam 1201 of thefirst variant of the twelfth embodiment.

The remaining description of the first variant of the twelfth embodimentis the same as corresponding portions of the description given for thetwelfth embodiment.

FIGS. 13a-13 b depict in schematic form an thirteenth embodiment of thepresent invention for measuring a linear displacement of an objectsubstantially independent of the orientation of the object and lateraldisplacements of the object, the lateral displacements being orthogonalto the linear displacement. The thirteenth embodiment is from a thirdclass of embodiments and variants thereof of the present inventionoperating in the first mode of operation for changes in orientation ofthe object about two orthogonal axes.

The description of light beam 1310 shown in FIG. 13a and of a source oflight beam 1310 is the same as the description of light beam 110 and ofa source of light beam 110 given for the first embodiment. As shown inFIG. 13a, beam 1310 is reflected by a surface of beam steering mirror1350 as beam 1312. Beam steering mirror 1350 and orientation transducers1351A and 1351B comprise a beam steering assembly that redirects theinput beam 1310. Beam 1312 is reflected by mirrors 1352A and 1352B toform beam 1314. Mirrors 1352A and 1352B together produce a constantdeviation between beams 1312 and 1314 in the plane of FIG. 13a, the sameas a penta prism.

Continuing with FIG. 13a, beam 1314 impinges on polarizing beam splitter1354 with a portion of beam 1314 transmitted as measurement beam 1315polarized in the plane of FIG. 13a and a second portion of beam 1314reflected as reference beam 1316 orthogonally polarized to the plane ofFIG. 13a.

Beam 1315 is transmitted by polarizing beam splitter 1356 as beam 1317.Beam 1317, polarized in the plane of FIG. 13a, is transmitted by aquarter wave phase retardation plate 1358 as a circularly polarizedbeam, reflected by measurement object mirror 1360, comprised of a singlereflecting surface, as a beam with opposite-handed circularpolarization, and subsequently transmitted by quarter wave phaseretardation plate 1358 as beam 1319 linearly polarized orthogonal to theplane of FIG. 13a. Beam 1319 is reflected by polarizing beam splitter1356 as beam 1321. Beam 1321 is reflected by mirror 1362 as beam 1323.

Reference beam 1316 is transmitted by half-wave phase retardation plate1364 as beam 1318 linearly polarized in the plane of FIG. 13a. Beam 1318is retroreflected by retroreflector 1366 as beam 1320.

Measurement beam 1323 is reflected by polarizing beam splitter 1368 asone component of output beam 1328. Beam 1320 is transmitted bypolarizing beam splitter 1368 as a second component of output beam 1328.Beam 1328 is reflected by mirror 1370 as beam 1330 and beam 1330 isreflected by a second surface of beam steering mirror 1350 as outputbeam 1332.

Output beam 1332 enters optical system 1398 and exits as beam 1338. Thereference beam component of output beam 1332 has experienced imageinversions about two orthogonal axes and the measurement beam componentof output beam 1332 has experienced an image inversion about one of theorthogonal axes. Optical system 1398 in FIG. 13a introduces anadditional image inversion in the measure beam component of beam 1332relative to the reference beam component of beam 1332 so that referencebeam component of beam 1338 has experienced the same number of imageinversions about for each of two orthogonal axes modulo two as number ofimage inversions experienced by the measurement beam component of beam1338.

The reference beam component of beam 1332 is transmitted by polarizingbeam splitter 1372 and reflected by mirror 1374 as beam 1334. Beam 1334enters image inverter 1376. As shown in FIG. 13b, image inverter 1376 iscomprised of three mirrors 1376A, 1376B, and 1376C. The plane of FIG.13b is orthogonal to the plane of FIG. 13a. Beam 1334 is reflected byeach of the three mirrors 1376A, 1376B, and 1376C and exits imageinverter 1376 as beam 1336.

The measurement beam component of beam 1332 is reflected by polarizingbeam splitter 1372 as beam 1333. Beam 1333 is reflected by mirrors 1378Aand 1378B to form beam 1335. Mirrors 1378A and 1378B together produce aconstant deviation between beams 1333 and 1333 in the plane of FIG. 13a,the same as a penta prism.

Beam 1336 is transmitted by polarizing beam splitter 1380 as onecomponent of output beam 1338. Beam 1335 is reflected by polarizing beamsplitter 1380 as a second component of output beam 1338.

A portion of beam 1338 is reflected by nonpolarizing beam splitter 1382as beam 1340. Beam 1340 impinges on detector 1384 to produce anelectrical signal 1348 related to any difference in the directions ofpropagation of measurement and reference beam components in beam 1340.

A second portion of output beam 1338 is transmitted by nonpolarizingbeam splitter 1382 as beam 1342. Beam 1342 is transmitted by a polarizer1386 orientated to produce a mixed optical beam 1344. Beam 1344 impingeson detector 1388 to generate electrical signal 1346 related to thedifference in phase of the measurement and reference beam components ofmixed beam 1344. Electrical signal 1346 is transmitted to electronicprocessor and computer 1390 for subsequent processing for informationabout the difference in phase of the measurement and reference beamcomponents of mixed beam 1344.

Electrical signal 1348 is transmitted to orientation transducers 1351Aand 1351B. Information contained in signal 1348 is used as the errorsignal in a servo system to alter the orientation of beam steeringmirror 1350 by orientation transducers 1351A and 1351B so as to maintainthe direction of propagation of the measurement beam component of beam1332 substantially constant relative to the direction of propagation ofthe reference beam component of beam 1332 independent of changes oforientation of measurement object mirror 1360. Under the condition thatthe physical path length of the measurement beam from the first surfaceof beam steering mirror 1350 to measurement object mirror 1360 is equalto the physical path length of the measurement beam from measurementobject mirror 1360 to the second surface of steering mirror 1350, therewill be no lateral shear of measurement beam component of beam 1332 atsteering mirror 1350 when changes in orientation of measurement objectmirror 1360 are compensated by changes in orientation of steering mirror1350. This condition with respect to physical path lengths will bereferenced as the thirteenth embodiment condition.

The remaining description of the thirteenth embodiment is the same ascorresponding portions of the descriptions given for precedingembodiments and variants thereof.

The thirteenth embodiment of the present invention exhibits an advantageadditional to those described for the first embodiment of the presentinvention. The additional advantage relates to the size of the beamsteering mirror 1350 and orientation transducers 1351A and 1351B. Underthe thirteenth embodiment condition, the size of beam steering mirror1350 need only be large enough to accommodate the projected size ofinput beam 1310, the location of input beam 1310 and output beam 1332being stationary at beam steering mirror 1350. This leads to anadditional advantage for the thirteenth embodiment with respect to areduced minimum size permitted for beam steering mirror 1350 andorientation transducers 1351A and 1351B and concomitant improvement infrequency response of beam steering mirror assembly.

For end use applications where the image inversion in beam 1332 does notpresent undesired complications, inverter 1398 of the thirteenthembodiment may be omitted to form a variant of the thirteenth embodimentwithout departing from the scope or spirit of the present invention. Theremaining description of the variant of the thirteenth embodiment is thesame as corresponding portions of the descriptions given for thethirteenth embodiment. Other variations of the thirteenth embodiment caninclude those in which the measurement object includes a roof prism or aretroreflector to reflect the measurement beam. In such variations thesystem can operate in the second mode as required to compensate forchanges in the position of the measurement object.

In some of the embodiments described above, the beam steering assemblyand control circuit can be used to reduce negative effects of spuriousreflections on the laser source generating the reference and measurementbeams. Spurious reflections resulting from imperfections in thepolarizing beam splitting surfaces or the quarter wave plates can, insome cases, reflect back to the laser source and destabilize or alterthe performance of the laser source. To reduce such negative effects,the system can be aligned as follows.

If spurious reflections that negatively affect the performance of thesource laser are identified, one of the optics that direct the referencebeam is adjusted to slightly alter the direction of the reference beamand produce a corresponding change in the direction or position of theexit reference beam. For example, if the system includes a plane mirroras a reference object, the angular orientation of the plane mirror canbe adjusted, or if the system includes mirrors that form a referenceretroreflector, these mirrors can be adjusted so that their surfaces arenot exactly normal to one another. Alternatively, a wedge or prism canbe introduced into the reference path to introduce a small angularoffset in the reference beam. Whatever the optic, that optic is adjusteduntil the spurious reflections derived from the reference beam no longerpropagate back to the laser source.

In addition, the detector in the control circuit detects the change inthe direction or position of the exit reference beam and causes theservo controller to reorient the beam steering mirror in response to achange in the relative position and/or direction of the exit referenceand measurement beams. The beam steering mirror redirects themeasurement beam such that the exit measurement beam tracks the changesto the exit reference beam, e.g., such that the exit reference andmeasurement beams remain substantially parallel and/or coextensive. As aresult, the spurious reflections derived from the measurement beamshould no longer propagate back to the laser source. Moreover, the beamsteering assembly and control circuit maintains the alignment of thereference and measurement beams.

This procedure can be implemented during fabrication of theinterferometry system to determine an alignment for the optics of thesystem that prevents or reduces propagation of spurious reflections backto the laser source. In general, the procedure optically introduces abias offset to the control circuit and its control of the orientation ofthe beam steering assembly. Alternatively, an offset can be introducedelectronically in the servo controller. However introduced, the offsetshould be large enough to prevent, or at least minimize, propagation ofspurious reflections back to the laser source over the typical range oforientations exercised by the beam steering element in response tochanges in the angular orientation or position of the measurementobject.

The interferometry systems described above can be especially useful inlithography applications used fabricating large scale integratedcircuits such as computer chips and the like. Lithography is the keytechnology driver for the semiconductor manufacturing industry. overlayimprovement is one of the five most difficult challenges down to andbelow 100 nm line widths (design rules), see for example theSemiconductor Industry Roadmap, p82 (1997). Overlay depends directly onthe performance, i.e. accuracy and precision, of the distance measuringinterferometers used to position the wafer and reticle (or mask) stages.Since a lithography tool may produce $50-100M/year of product, theeconomic value from improved performance distance measuringinterferometers is substantial. Each 1% increase in yield of thelithography tool results in approximately $1M/year economic benefit tothe integrated circuit manufacturer and substantial competitiveadvantage to the lithography tool vendor.

The function of a lithography tool is to direct spatially patternedradiation onto a photoresist-coated wafer. The process involvesdetermining which location of the wafer is to receive the radiation(alignment) and applying the radiation to the photoresist at thatlocation (exposure).

To properly position the wafer, the wafer includes alignment marks onthe wafer that can be measured by dedicated sensors. The measuredpositions of the alignment marks define the location of the wafer withinthe tool. This information, along with a specification of the desiredpatterning of the wafer surface, guides the alignment of the waferrelative to the spatially patterned radiation. Based on suchinformation, a translatable stage supporting the photoresist-coatedwafer moves the wafer such that the radiation will expose the correctlocation of the wafer.

During exposure, a radiation source illuminates a patterned reticle,which scatters the radiation to produce the spatially patternedradiation. The reticle is also referred to as a mask, and these termsare used interchangeably below. In the case of reduction lithography, areduction lens collects the scattered radiation and forms a reducedimage of the reticle pattern. Alternatively, in the case of proximityprinting, the scattered radiation propagates a small distance (typicallyon the order of microns) before contacting the wafer to produce a 1:1image of the reticle pattern. The radiation initiates photo-chemicalprocesses in the resist that convert the radiation pattern into a latentimage within the resist.

The interferometry systems described above are important components ofthe positioning mechanisms that control the position of the wafer andreticle, and register the reticle image on the wafer.

In general, the lithography system, also referred to as an exposuresystem, typically includes an illumination system and a waferpositioning system. The illumination system includes a radiation sourcefor providing radiation such as ultraviolet, visible, x-ray, electron,or ion radiation, and a reticle or mask for imparting the pattern to theradiation, thereby generating the spatially patterned radiation. Inaddition, for the case of reduction lithography, the illumination systemcan include a lens assembly for imaging the spatially patternedradiation onto the wafer. The imaged radiation exposes resist coatedonto the wafer. The illumination system also includes a mask stage forsupporting the mask and a positioning system for adjusting the positionof the mask stage relative to the radiation directed through the mask.The wafer positioning system includes a wafer stage for supporting thewafer and a positioning system for adjusting the position of the waferstage relative to the imaged radiation. Fabrication of integratedcircuits can include multiple exposing steps. For a general reference onlithography, see, for example, J. R. Sheats and B. W. Smith, inMicrolithography: Science and Technology (Marcel Dekker, Inc., New York,1998), the contents of which is incorporated herein by reference.

The interferometry systems described above can be used to preciselymeasure the positions of each of the wafer stage and mask stage relativeto other components of the exposure system, such as the lens assembly,radiation source, or support structure. In such cases, theinterferometry system can be attached to a stationary structure and themeasurement object attached to a movable element such as one of the maskand wafer stages. Alternatively, the situation can be reversed, with theinterferometry system attached to a movable object and the measurementobject attached to a stationary object.

More generally, the interferometry systems can be used to measure theposition of any one component of the exposure system relative to anyother component of the exposure system, in which the interferometrysystem is attached to, or supported by, one of the components and themeasurement object is attached, or is supported by the other of thecomponents.

An example of a lithography scanner 1400 using an interferometry system1426 is shown in FIG. 14a. The interferometry system is used toprecisely measure the position of a wafer within an exposure system.Here, stage 1422 is used to position the wafer relative to an exposurestation. Scanner 1400 comprises a frame 1402, which carries othersupport structures and various components carried on those structures.An exposure base 1404 has mounted on top of it a lens housing 1406 atopof which is mounted a reticle or mask stage 1416, which is used tosupport a reticle or mask. A positioning system for positioning the maskrelative to the exposure station is indicated schematically by element1417. Positioning system 1417 can include, e.g., piezoelectrictransducer elements and corresponding control electronics. Although, itis not included in this described embodiment, one or more of theinterferometry systems described above can also be used to preciselymeasure the position of the mask stage as well as other moveableelements whose position must be accurately monitored in processes forfabricating lithographic structures (see supra Sheats and SmithMicrolithograpy: Science and Technology).

Suspended below exposure base 1404 is a support base 1413 that carrieswafer stage 1422. Stage 1422 includes a plane mirror for reflecting ameasurement beam 1454 directed to the stage by interferometry system1426. A positioning system for positioning stage 1422 relative tointerferometry system 1426 is indicated schematically by element 1419.Positioning system 1419 can include, e.g., piezoelectric transducerelements and corresponding control electronics. The measurement beamreflects back to the interferometry system, which is mounted on exposurebase 1404. The interferometry system can be any of the embodimentsdescribed previously.

During operation, a radiation beam 1410, e.g., an ultraviolet (UV) beamfrom a UV laser (not shown), passes through a beam shaping opticsassembly 1412 and travels downward after reflecting from mirror 1414.Thereafter, the radiation beam passes through a mask (not shown) carriedby mask stage 1416. The mask (not shown) is imaged onto a wafer (notshown) on wafer stage 1422 via a lens assembly 1408 carried in a lenshousing 1406. Base 1404 and the various components supported by it areisolated from environmental vibrations by a damping system depicted byspring 1420.

In other embodiments of the lithographic scanner, one or more of theinterferometry systems described previously can be used to measuredistance along multiple axes and angles associated for example with, butnot limited to, the wafer and reticle (or mask) stages. Also, ratherthan a UV laser beam, other beams can be used to expose the waferincluding, e.g., x-ray beams, electron beams, ion beams, and visibleoptical beams.

In addition, the lithographic scanner can include a column reference inwhich interferometry system 1426 directs the reference beam to lenshousing 1406 or some other structure that directs the radiation beamrather than a reference path internal to the interferometry system. Theinterference signal produce by interferometry system 1426 when combiningmeasurement beam 1454 reflected from stage 1422 and the reference beamreflected from lens housing 1406 indicates changes in the position ofthe stage relative to the radiation beam. Furthermore, in otherembodiments the interferometry system 1426 can be positioned to measurechanges in the position of reticle (or mask) stage 1416 or other movablecomponents of the scanner system. Finally, the interferometry systemscan be used in a similar fashion with lithography systems involvingsteppers, in addition to, or rather than, scanners.

As is well known in the art, lithography is a critical part ofmanufacturing methods for making semiconducting devices. For example,U.S. Pat. No. 5,483,343 outlines steps for such manufacturing methods.These steps are described below with reference to FIGS. 14b and 14 c.FIG. 14b is a flow chart of the sequence of manufacturing asemiconductor device such as a semiconductor chip (e.g. IC or LSI), aliquid crystal panel or a CCD. Step 1451 is a design process fordesigning the circuit of a semiconductor device. Step 1452 is a processfor manufacturing a mask on the basis of the circuit pattern design.Step 1453 is a process for manufacturing a wafer by using a materialsuch as silicon.

Step 1454 is a wafer process which is called a pre-process wherein, byusing the so prepared mask and wafer, circuits are formed on the waferthrough lithography. Step 1455 is an assembling step, which is called apost-process wherein the wafer processed by step 1454 is formed intosemiconductor chips. This step includes assembling (dicing and bonding)and packaging (chip sealing). Step 1456 is an inspection step whereinoperability check, durability check and so on of the semiconductordevices produced by step 1455 are carried out. With these processes,semiconductor devices are finished and they are shipped (step 1457).

FIG. 14c is a flow chart showing details of the wafer process. Step 1461is an oxidation process for oxidizing the surface of a wafer. Step 1462is a CVD process for forming an insulating film on the wafer surface.Step 1463 is an electrode forming process for forming electrodes on thewafer by vapor deposition. Step 1464 is an ion implanting process forimplanting ions to the wafer. Step 1465 is a resist process for applyinga resist (photosensitive material) to the wafer. Step 1466 is anexposure process for printing, by exposure, the circuit pattern of themask on the wafer through the exposure apparatus described above. Step1467 is a developing process for developing the exposed wafer. Step 1468is an etching process for removing portions other than the developedresist image. Step 1469 is a resist separation process for separatingthe resist material remaining on the wafer after being subjected to theetching process. By repeating these processes, circuit patterns areformed and superimposed on the wafer.

The interferometry systems described above can also be used in otherapplications in which the relative position of an object needs to bemeasured precisely. For example, in applications in which a write beamsuch as a laser, x-ray, ion, or electron beam, marks a pattern onto asubstrate as either the substrate or beam moves, the interferometrysystems can be used to measure the relative movement between thesubstrate and write beam.

As an example, a schematic of a beam writing system 1600 is shown inFIG. 16. A source 1610 generates a write beam 1612, and a beam focusingassembly 1614 directs the radiation beam to a substrate 1616 supportedby a movable stage 1618. To determine the relative position of thestage, an interferometry system 1620 directs a reference beam 1622 to amirror 1624 mounted on beam focusing assembly 1614 and a measurementbeam 1626 to a mirror 1628 mounted on stage 1618. Interferometry system1620 can be any of the interferometry systems described previously.Changes in the position measured by the interferometry system correspondto changes in the relative position of write beam 1612 on substrate1616. Interferometry system 1620 sends a measurement signal 1632 tocontroller 1630 that is indicative of the relative position of writebeam 1612 on substrate 1616. Controller 1630 sends an output signal 1634to a base 1636 that supports and positions stage 1618. In addition,controller 1630 sends a signal 1638 to source 1610 to vary the intensityof, or block, write beam 1612 so that the write beam contacts thesubstrate with an intensity sufficient to cause photophysical orphotochemical change only at selected positions of the substrate.Furthermore, in some embodiments, controller 1630 can cause beamfocusing assembly 1614 to scan the write beam over a region of thesubstrate, e.g., using signal 1644. As a result, controller 1630 directsthe other components of the system to pattern the substrate. Thepatterning is typically based on an electronic design pattern stored inthe controller. In some applications the write beam patterns a resistcoated on the substrate and in other applications the write beamdirectly patterns, e.g., etches, the substrate.

An important application of such a system is the fabrication of masksand reticles used in the lithography methods described previously. Forexample, to fabricate a lithography mask an electron beam can be used topattern a chromium-coated glass substrate. In such cases where the writebeam is an electron beam, the beam writing system encloses the electronbeam path in a vacuum. Also, in cases where the write beam is, e.g., anelectron or ion beam, the beam focusing assembly includes electric fieldgenerators such as quadrapole lenses for focusing and directing thecharged particles onto the substrate under vacuum. In other cases wherethe write beam is a radiation beam, e.g., x-ray, UV, or visibleradiation, the beam focusing assembly includes corresponding optics andfor focusing and directing the radiation to the substrate.

It is understood that while the invention has been described inconjunction with the detailed description thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims.

For example, although embodiments described above use heterodynedetection to determine changes in the optical path length differencebetween the reference and measurement paths, the interferometry systemscan also use homodyne detection, in which the frequencies of thereference and measurement beams are the same. In such cases, anon-polarizing beam splitter splits the output beam formed by theoverlapping exit reference and measurement beams into two components,one of which passes through a quarter wave plate. Thereafter, each ofthe two components passes through a polarizer to produce a pair of mixedoptical signals, which are measured by a pair of detectors,respectively. The electronic signals measured by the detectors are inphase quadrature with one another and can be processed by a computer todetermine the changes in the optical path length difference usingwell-known homodyne processing techniques.

Furthermore, in other embodiments in which the reference beam reflectsfrom a reference object, the interferometry system can include one ormore additional beam steering assemblies (and corresponding controllersand detectors) that redirect the reference beam in response to changesin the angular orientation or position of the reference object.

Finally, in other embodiments the beam steering assembly can bedifferent from a mirror oriented by a positioning system. For example,the beam steering assembly can include one or more of the followingelements: an electro-optic or acousto-optic beam deflector, a pair ofrotating optical wedges (Risely prisms), a variable wedge element, andany other such element that can alter the direction of a light beam.

Other aspects, advantages, and modifications are within the scope of thefollowing claims.

What is claimed is:
 1. A distance measuring interferometry systemcomprising: an interferometer which during operation directs a referencebeam along a reference path and a measurement beam along a measurementpath contacting a measurement object and combines the reference andmeasurement beams to produce overlapping exit reference and measurementbeams, the overlapping exit reference and measurement beams indicativeof changes in a relative optical path length between the reference andmeasurement paths, the interferometer comprising an optical elementpositioned to receive and direct at least one of the reference andmeasurement beams and a beam steering assembly having a beam steeringelement and a positioning system to orient the beam steering elementrelative to the optical element, the beam steering element positioned todirect the reference and measurement beams, the reference andmeasurement beams contacting the beam steering element; and a controlcircuit which during operation causes the positioning system to reorientthe beam steering element in response to changes in at least one ofangular orientation and position of the measurement object.
 2. Thesystem of claim 1, wherein during operation the interferometer furtherseparates an input beam into a pair of spatially separated beams todefine the reference and measurement beams.
 3. The system of claim 2,wherein the beam steering element comprises a polarizing beam splitterthat separates the input beam into the reference and measurement beams,and the positioning system comprises at least one transducer mounted tothe polarizing beam splitter and operative to adjust an orientation ofthe polarizing beam splitter in response to a control signal from thecontrol circuit.
 4. The system of claim 2, wherein the interferometerfurther comprises a polarizing beam splitter that separates the inputbeam into the reference and measurement beams, and wherein the beamsteering element comprises a beam steering mirror and the positioningsystem comprises at least one transducer mounted to the beam steeringmirror and operative to adjust an orientation of the beam steeringmirror in response to a control signal from the control circuit.
 5. Thesystem of claim 4, wherein the optical element is a referenceretroreflector and the interferometer further comprises: a measurementretroreflector; and a reflective reference object, wherein duringoperation the polarizing beam splitter directs the reference beam to thereference object and the measurement beam to the measurementretroreflector, receives the reference beam from the reference objectand the measurement beam from the measurement retroreflector, and thendirects the reference and measurement beams to the beam steering mirror,at which time the beam steering mirror directs the reference beam to thereference retroreflector and the measurement beam to the measurementobject, receives the reference beam from the reference retroreflectorand the measurement beam from the measurement object, and then directsthe reference and measurement beams back to the polarizing beamsplitter, at which time the polarizing beam splitter combines thereference and measurement beams to produce the overlapping exitreference and measurement beams.
 6. The system of claim 5, wherein themeasurement retroreflector has an aperture through which the referencebeam travels to and from the reference object and the referenceretroreflector has an aperture through which the measurement beamtravels to and from the measurement object.
 7. The system of claim 6,wherein the interferometer further comprises a reference quarter waveplate through which the reference beam travels to and from the referenceobject and a measurement quarter wave plate through which themeasurement beam travels to and from the measurement object.
 8. Thesystem of claim 1, wherein during operation the interferometer receivesa pair of spatially separated input beams to define the reference andmeasurement beams.
 9. The system of claim 8, wherein the beam steeringelement comprises a polarizing beam splitter that directs the referencebeam along the reference path and the measurement beam along themeasurement path, and the positioning system comprises at least onetransducer mounted to the polarizing beam splitter and operative toadjust an orientation of the polarizing beam splitter in response to acontrol signal from the control circuit.
 10. The system of claim 8,wherein the interferometer further comprises a polarizing beam splitterthat directs the reference beam along the reference path and themeasurement beam along the measurement path, and wherein the beamsteering element comprises a beam steering mirror and the positioningsystem comprises at least one transducer mounted to the beam steeringmirror and operative to adjust an orientation of the beam steeringmirror in response to a control signal from the control circuit.
 11. Thesystem of claim 1, wherein the interferometer is a single-passinterferometer in which the measurement beam contacts the measurementobject only a single time.
 12. The system of claim 1, wherein duringoperation the control circuit causes the positioning system to reorientthe beam steering element so as to maintain the exit reference andmeasurement beams substantially parallel to one another over a range ofangular orientations of the measurement object.
 13. The system of claim1, wherein during operation the control circuit causes the positioningsystem to reorient the beam steering element to reduce changes indirection of the exit measurement beam caused by changes in the angularorientation of the measurement object.
 14. The system of claim 1,wherein during operation the control circuit causes the positioningsystem to reorient the beam steering element to reduce a transversedisplacement between the exit reference and measurement beams caused bychanges in the angular orientation of the measurement object.
 15. Thesystem of claim 1, wherein during operation the control circuit causesthe positioning system to reorient the beam steering element to reduce atransverse displacement of the measurement beam caused by changes in theangular orientation of the measurement object.
 16. The system of claim1, wherein during operation the control circuit causes the positioningsystem to reorient the beam steering element based on a signal generatedfrom the exit measurement beam.
 17. The system of claim 1, whereinduring operation the control circuit causes the positioning system toreorient the beam steering element such that the measurement beamcontacts the measurement object at substantially normal incidence over arange of angular orientations of the measurement object.
 18. The systemof claim 1, wherein during operation the control circuit causes thepositioning system to reorient the beam steering element to reduce atransverse displacement between the exit reference and measurement beamscaused by translations of the measurement object.
 19. The system ofclaim 1, wherein during operation the control circuit causes thepositioning system to reorient the beam steering element to reduce atransverse displacement of the measurement beam caused by translationsof the measurement object.
 20. The system of claim 1, wherein themeasurement object is a plane mirror and during operation themeasurement beam contacts the measurement object at non-normal incidenceand the control circuit causes the positioning system to reorient thebeam steering element to reduce transverse displacements between theexit reference and measurement beams caused by translations of themeasurement object.
 21. The system of claim 1, further comprising apolarizer which during operation receives the exit reference andmeasurement beams and mixes polarizations of the exit reference andmeasurement beams to produce a mixed beam having a phase indicative ofthe changes in the optical path length difference between the referenceand measurement paths.
 22. The system of claim 1, wherein the controlcircuit comprises a detector having spatially resolved detector elementsoperative to measure positions and/or directions of the exit referenceand measurement beams and generate a measurement signal indicative ofthe positions and/or directions, and a controller operative to receivethe measurement signal from the detector and send a control signal basedon the measurement signal to the positioning system, the control signalcausing the positioning system to reorient the beam steering element.23. The system of claim 1, wherein a translation of the beam steeringelement does not introduce a substantial change in the relative opticalpath length between the reference and measurement paths.
 24. Alithography system for use in fabricating integrated circuits on awafer, the system comprising: a stage for supporting the wafer; anillumination system for imaging spatially patterned radiation onto thewafer; a positioning system for adjusting the position of the stagerelative to the imaged radiation; and the interferometry system of claim1 for measuring the position of the wafer relative to the imagedradiation.
 25. A lithography system for use in fabricating integratedcircuits on a wafer, the system comprising: a stage for supporting thewafer; and an illumination system including a radiation source, a mask,a positioning system, a lens assembly, and the interferometry system ofclaim 1, wherein during operation the source directs radiation throughthe mask to produce spatially patterned radiation, the positioningsystem adjusts the position of the mask relative to the radiation fromthe source, the lens assembly images the spatially patterned radiationonto the wafer, and the interferometry system measures the position ofthe mask relative to the radiation from the source.
 26. A lithographysystem for fabricating integrated circuits comprising first and secondcomponents, the first and second components being movable relative toeach other, and the interferometry system of claim 1, wherein the firstcomponents comprises the measurement object and the interferometrysystem monitors the position of the first component relative to thesecond component.
 27. A beam writing system for use in fabricating alithography mask, the system comprising: a source providing a write beamto pattern a substrate; a stage supporting the substrate; a beamdirecting assembly for delivering the write beam to the substrate; apositioning system for positioning the stage and beam directing assemblyrelative one another; and the inteferometry system of claim 1 formeasuring the position of the stage relative to the beam directingassembly.
 28. A distance measuring interferometry system comprising: aninterferometer which during operation receives at least one input beamto define a reference beam and a measurement beam, directs the referencebeam along a reference path and the measurement beam along a measurementpath contacting a measurement object, and combines the reference andmeasurement beams to produce an output beam comprising overlapping exitreference and measurement beams, the output beam indicative of changesin a relative optical path length between the reference and measurementpaths; and a beam steering assembly comprising a beam steering elementand a positioning system to orient the beam steering element, duringoperation the beam steering element directs the at least one input andthe output beam, the at least one input and the output beam contactingthe beam steering element; and a control circuit which during operationcauses the positioning system to reorient the beam steering element inresponse to changes in at least one of angular orientation and positionof the measurement object.
 29. The interferometry system of claim 28,wherein the at least one input beam is a single input beam and theinterferometer separates the single input beam into a pair of spatiallyseparated beams to define the reference and measurement beams.
 30. Theinterferometry system of claim 28, wherein the at least one input beamis a pair of spatially separated input beams.
 31. The system of claim28, wherein the beam steering element comprises a beam steering mirror,and the positioning system comprises at least one transducer mounted tothe beam steering mirror and operative to adjust an orientation of thebeam steering mirror in response to a control signal from the controlcircuit.
 32. The system of claim 31, wherein the beam steering mirrorhas front and back faces, and wherein during operation the input beamcontacts the front face of the beam steering mirror and the output beamcontacts the back face of the beam steering mirror.
 33. The system ofclaim 28, wherein the interferometer is a single-pass interferometer inwhich the measurement beam contacts the measurement object only a singletime.
 34. The system of claim 28, wherein during operation the controlcircuit causes the positioning system to reorient the beam steeringelement so as to maintain the exit reference and measurement beamssubstantially parallel to one another over a range of angularorientations of the measurement object.
 35. The system of claim 28,wherein during operation the control circuit causes the positioningsystem to reorient the beam steering element to reduce changes indirection of the exit measurement beam caused by changes in the angularorientation of the measurement object.
 36. The system of claim 28,wherein during operation the control circuit causes the positioningsystem to reorient the beam steering element to reduce a transversedisplacement between the exit reference and measurement beams caused bychanges in angular orientation of the measurement object.
 37. The systemof claim 28, wherein during operation the control circuit causes thepositioning system to reorient the beam steering element to reduce atransverse displacement of the measurement beam caused by changes inangular orientation of the measurement object.
 38. The system of claim28, wherein during operation the control circuit causes the positioningsystem to reorient the beam steering element to reduce a transversedisplacement between the exit reference and measurement beams caused bya translation of the measurement object.
 39. The system of claim 28,wherein during operation the control circuit causes the positioningsystem to reorient the beam steering element to reduce a transversedisplacement of the measurement beam caused by a translation of themeasurement object.
 40. The system of claim 28, wherein during operationthe control circuit causes the positioning system to reorient the beamsteering element such that the measurement beam contacts the measurementobject at substantially normal incidence over a range of angularorientations of the measurement object.
 41. The system of claim 28,wherein a translation of the beam steering element does not introduce asubstantial change in the relative optical path length between thereference and measurement paths.
 42. A lithography system for use infabricating integrated circuits on a wafer, the system comprising: astage for supporting the wafer; an illumination system for imagingspatially patterned radiation onto the wafer; a positioning system foradjusting the position of the stage relative to the imaged radiation;and the interferometry system of claim 28 for measuring the position ofthe wafer relative to the imaged radiation.
 43. A lithography system foruse in fabricating integrated circuits on a wafer, the systemcomprising: a stage for supporting the wafer; and an illumination systemincluding a radiation source, a mask, a positioning system, a lensassembly, and the interferometry system of claim 28, wherein duringoperation the source directs radiation through the mask to producespatially patterned radiation, the positioning system adjusts theposition of the mask relative to the radiation from the source, the lensassembly images the spatially patterned radiation onto the wafer, andthe interferometry system measures the position of the mask relative tothe radiation from the source.
 44. A lithography system for fabricatingintegrated circuits comprising first and second components, the firstand second components being movable relative to each other, and theinterferometry system of claim 28, wherein the first componentscomprises the measurement object and the interferometry system monitorsthe position of the first component relative to the second component.45. A beam writing system for use in fabricating a lithography mask, thesystem comprising: a source providing a write beam to pattern asubstrate; a stage supporting the substrate; a beam directing assemblyfor delivering the write beam to the substrate; a positioning system forpositioning the stage and beam directing assembly relative one another;and the inteferometry system of claim 28 for measuring the position ofthe stage relative to the beam directing assembly.
 46. A method forinterferometry comprising: directing a reference beam along a referencepath and a measurement beam along a measurement path contacting ameasurement object; combining the reference and measurement beams toform overlapping exit reference and measurement beams, the overlappingexit measurement and reference beams indicative of changes in a relativeoptical path length between the reference and measurement paths; andusing an electronic control system to redirect the measurement beam andthe reference beam in response to changes in angular orientation of themeasurement object.
 47. A method for interferometry comprising:receiving at least one input beam to define a reference beam and ameasurement beam; directing the reference beam along a reference pathand the measurement beam along a measurement path contacting ameasurement object; combining the reference and measurement beams toform an output beam comprising overlapping exit reference andmeasurement beams, the overlapping exit measurement and reference beamsindicative of changes in a relative optical path length between thereference and measurement paths; and using an electronic control systemto redirect the at least one input beam and the output beam in responseto changes in at least one of angular orientation and position of themeasurement object.
 48. A lithography method for use in fabricatingintegrated circuits comprising: imaging spatially patterned radiationonto a wafer; positioning the wafer relative to the imaged radiation;and measuring the position of the wafer relative to the imaged radiationusing the interferometry method of claim 47, wherein one of a stagesupporting the wafer and an illumination system producing the radiationincludes the measurement object.
 49. A lithography method for use in thefabrication of integrated circuits comprising: directing input radiationthrough a mask to produce spatially patterned radiation; positioning themask relative to the input radiation; measuring the position of the maskrelative to the input radiation using the interferometry method of claim47, wherein one of a stage supporting the mask and a illumination systemproviding the input radiation include the measurement object; andimaging the spatially patterned radiation onto a wafer.
 50. Alithography method comprising: positioning a first component of alithography system relative to a second component of a lithographysystem to expose a wafer to spatially patterned radiation; and measuringthe position of the first component relative to the second componentusing the method of claim 47, wherein the first component includes themeasurement object.
 51. A beam writing method for use in fabricating alithography mask, the method comprising: directing a write beam to asubstrate to pattern the substrate; positioning the substrate relativeto the write beam; and measuring the position of the substrate relativeto the write beam using the interferometry method of claim
 47. 52. Alithography method for use in fabricating integrated circuitscomprising: imaging spatially patterned radiation onto a wafer;positioning the wafer relative to the imaged radiation; and measuringthe position of the wafer relative to the imaged radiation using aninterferometry method, wherein one of a stage supporting the wafer andan illumination system producing the radiation includes a measurementobject; and wherein the interferometry method includes directing areference beam along a reference path and a measurement beam along ameasurement path contacting the measurement object, combining thereference and measurement beams to form overlapping exit reference andmeasurement beams, the overlapping exit measurement and reference beamsindicative of changes in a relative optical path length between thereference and measurement paths, and using an electronic control systemto redirect the measurement beam and the reference beam in response tochanges in at least one of angular orientation and position of themeasurement object.
 53. A lithography method for use in the fabricationof integrated circuits comprising: directing input radiation through amask to produce spatially patterned radiation; positioning the maskrelative to the input radiation; measuring the position of the maskrelative to the input radiation using an interferometry method, whereinone of a stage supporting the mask and a illumination system providingthe input radiation include a measurement object; and imaging thespatially patterned radiation onto a wafer, wherein the interferometrymethod includes directing a reference beam along a reference path and ameasurement beam along a measurement path contacting the measurementobject, combining the reference and measurement beams to formoverlapping exit reference and measurement beams, the overlamping exitmeasurement and reference beams indicative of changes in a relativeoptical path length between the reference and measurement paths, andusing an electronic control system to redirect the measurement beam andthe reference beam in response to changes in at least one of angularorientation and position of the measurement object.
 54. A lithographymethod comprising: positioning a first component of a lithography systemrelative to a second component of a lithography system to expose a waferto spatially patterned radiation; and measuring the position of thefirst component relative to the second component using an interferometrymethod wherein the first component includes a measurement object, andthe interferometry method includes directing a reference beam along areference path and a measurement beam along a measurement pathcontacting the measurement object, combining the reference andmeasurement beams to form overlapping exit reference and measurementbeams, the overlapping exit measurement and reference beams indicativeof changes in a relative optical path length between the reference andmeasurement paths, and using an electronic control system to redirectthe measurement beam and the reference beam in response to changes in atleast one of angular orientation and position of the measurement object.55. A beam writing method for use in fabricating a lithography mask, themethod comprising: directing a write beam to a substrate to pattern thesubstrate; positioning the substrate relative to the write beam; andmeasuring the position of the substrate relative to the write beam usingan interferometry method including directing a reference beam along areference path and a measurement beam along a measurement pathcontacting the measurement object, combining the reference andmeasurement beams to form overlapping exit reference and measurementbeams, the overlapping exit measurement and reference beams indicativeof changes in a relative optical path length between the reference andmeasurement paths, and using an electronic control system to redirectthe measurement beam and the reference beam in response to changes in atleast one of angular orientation and position of the measurement object.56. A distance measuring interferometry system comprising: aninterferometer which during operation receives a pair of spatiallyseparated input beams to define a reference beam and a measurement beam,directs the reference beam along a reference path and the measurementbeam along a measurement path contacting a measurement object, andcombines the reference and measurement beams to produce overlapping exitreference and measurement beams, the overlapping exit reference andmeasurement beams indicative of changes in a relative optical pathlength between the reference and measurement paths, the interferometercomprising a beam steering assembly having a beam steering element and apositioning system to orient the beam steering element, the beamsteering element positioned to direct at least one of the reference andmeasurement beams, the at least one of the reference and measurementbeams contacting the beam steering element; and a control circuit whichduring operation causes the positioning system to reorient the beamsteering element in response to changes in at least one of angularorientation and position of the measurement object.
 57. The system ofclaim 56, wherein the beam steering element comprises a polarizing beamsplitter that directs the reference beam along the reference path andthe measurement beam along the measurement path, and the positioningsystem comprises at least one transducer mounted to the polarizing beamsplitter and operative to adjust an orientation of the polarizing beamsplitter in response to a control signal from the control circuit. 58.The system of claim 56, wherein the interferometer further comprises apolarizing beam splitter that directs the reference beam along thereference path and the measurement beam along the measurement path, andwherein the beam steering element comprises a beam steering mirror andthe positioning system comprises at least one transducer mounted to thebeam steering mirror and operative to adjust an orientation of the beamsteering mirror in response to a control signal from the controlcircuit.
 59. The system of claim 56, further comprising a polarizerwhich during operation receives the exit reference and measurement beamsand mixes polarizations of the exit reference and measurement beams toproduce a mixed beam having a phase indicative of the changes in theoptical path length difference between the reference and measurementpaths.
 60. A distance measuring interferometry system comprising: aninterferometer which during operating separates in an input beam into areference beam and a measurement beam, directs the reference beam alonga reference path and the measurement beam along a measurement pathcontacting a measurement object and combines the reference andmeasurement beams to produce overlapping exit reference and measurementbeams, the overlapping exit reference and measurement beams indicativeof changes in a relative optical path length between the reference andmeasurement paths, the interferometer comprising a beam steeringassembly having a beam steering element and a positioning system toorient the beam steering element, the beam steering element positionedto direct at least one of the reference and measurement beams, the atleast one of the reference and measurement beams contacting the beamsteering element; and a control circuit which during operation causesthe positioning system to reorient the beam steering element in responseto changes in at least one of angular orientation and position of themeasurement object, wherein the interferometer further comprises apolarizing beam splitter that separates the input beam into thereference and measurement beams, wherein the beam steering elementcomprises a beam steering mirror and the positioning system comprises atleast one transducer mounted to the beam steering mirror and operativeto adjust an orientation of the beam steering mirror in response to acontrol signal from the control circuit, wherein the beam steeringmirror has front and back faces, and wherein during operation the frontface of the beam steering mirror directs the measurement beam to themeasurement object and the back face of the beam steering mirrorreceives the measurement beam from the measurement object.
 61. Thesystem of claim 60, wherein the interferometer further comprises asecond polarizing beam splitter which during operation receives thereference beam from the reference path and the measurement beam from theback face of the beam steering mirror and produces the exit referenceand measurement beams.
 62. A distance measuring interferometry systemcomprising: an interferometer which during operation directs a referencebeam along a reference path and a measurement beam along a measurementpath contacting a measurement object and combines the reference andmeasurement beams to produce overlapping exit reference and measurementbeams, the overlapping exit reference and measurement beams indicativeof changes in a relative optical path length between the reference andmeasurement paths, the interferometer comprising a beam steeringassembly having a beam steering element and a positioning system toorient the beam steering element, the beam steering element positionedto direct at least one of the reference and measurement beams, the atleast one of the reference and measurement beams contacting the beamsteering element; and a control circuit which during operation causesthe positioning system to reorient the beam steering element in responseto changes in at least one of angular orientation and position of themeasurement object, wherein the interferometer further comprises atleast one additional beam steering assembly comprising an additionalbeam steering element and an additional positioning system to orient theadditional beam steering element, wherein during operation theadditional beam steering element directs the measurement beam, themeasurement beam contacting the additional beam steering element, andwherein during operation the control circuit causes the positioningsystems to reorient the beam steering elements to optimize the overlapof the exit reference and measurement beams and maintain the exitreference and measurement beams substantially parallel over a range ofangular orientations and positions of the measurement object.
 63. Thesystem of claim 62, wherein the control circuit comprises two detectorshaving spatially resolved detector elements operative to measure aposition and direction of the exit measurement beam and generatemeasurement signals indicative of the position and direction, and acontroller operative to receive the measurement signals from thedetectors and send control signals to the positioning systems based onthe signals from the detectors, the control signals causing thepositioning systems to reorient the beam steering elements.
 64. Alithography system for use in fabricating integrated circuits on awafer, the system comprising: a stage for supporting the wafer; anillumination system for imaging spatially patterned radiation onto thewafer; a positioning system for adjusting the position of the stagerelative to the imaged radiation; and an interferometry system formeasuring the position of the wafer relative to the imaged radiation,the interferometry system comprising: an interferometer which duringoperation directs a reference beam along a reference path and ameasurement beam along a measurement path contacting a measurementobject and combines the reference and measurement beams to produceoverlapping exit reference and measurement beams, the overlapping exitreference and measurement beams indicative of changes in a relativeoptical path length between the reference and measurement paths, theinterferometer comprising a beam steering assembly having a beamsteering element and a positioning system to orient the beam steeringelement, the beam steering element positioned to direct at least one ofthe reference and measurement beams, the at least one of the referenceand measurement beams contacting the beam steering element; and acontrol circuit which during operation causes the positioning system toreorient the beam steering element in response to changes in at leastone of angular orientation and position of the measurement object.
 65. Alithography system for use in fabricating integrated circuits on awafer, the system comprising: a stage for supporting the wafer; and anillumination system including a radiation source, a mask, a positioningsystem, a lens assembly, and an interferometry system, wherein duringoperation the source directs radiation through the mask to producespatially patterned radiation, the positioning system adjusts theposition of the mask relative to the radiation from the source, the lensassembly images the spatially patterned radiation onto the wafer, andthe interferometry system measures the position of the mask relative tothe radiation from the source, the interferometry system comprising: aninterferometer which during operation directs a reference beam along areference path and a measurement beam along a measurement pathcontacting a measurement object and combines the reference andmeasurement beams to produce overlapping exit reference and measurementbeams, the overlapping exit reference and measurement beams indicativeof changes in a relative optical path length between the reference andmeasurement paths, the interferometer comprising a beam steeringassembly having a beam steering element and a positioning system toorient the beam steering element, the beam steering element positionedto direct at least one of the reference and measurement beams, the atleast one of the reference and measurement beams contacting the beamsteering element; and a control circuit which during operation causesthe positioning system to reorient the beam steering element in responseto changes in at least one of angular orientation and position of themeasurement object.
 66. A beam writing system for use in fabricating alithography mask, the system comprising: a source providing a write beamto pattern a substrate; a stage supporting the substrate; a beamdirecting assembly for delivering the write beam to the substrate; apositioning system for positioning the stage and beam directing assemblyrelative one another; and an inteferometry system for measuring theposition of the stage relative to the beam directing assembly, theinterferometry system comprising: an interferometer which duringoperation directs a reference beam along a reference path and ameasurement beam along a measurement path contacting a measurementobject and combines the reference and measurement beams to produceoverlapping exit reference and measurement beams, the overlapping exitreference and measurement beams indicative of changes in a relativeoptical path length between the reference and measurement paths, theinterferometer comprising a beam steering assembly having a beamsteering element and a positioning system to orient the beam steeringelement, the beam steering element positioned to direct at least one ofthe reference and measurement beams, the at least one of the referenceand measurement beams contacting the beam steering element; and acontrol circuit which during operation causes the positioning system toreorient the beam steering element in response to changes in at leastone of angular orientation and position of the measurement object.
 67. Alithography method for use in fabricating integrated circuitscomprising: imaging spatially patterned radiation onto a wafer;positioning the wafer relative to the imaged radiation; and measuringthe position of the wafer relative to the imaged radiation using aninterferometry method comprising: directing a reference beam along areference path and a measurement beam along a measurement pathcontacting a measurement object; combining the reference and measurementbeams to form overlapping exit reference and measurement beams, theoverlapping exit measurement and reference beams indicative of changesin a relative optical path length between the reference and measurementpaths; and using an electronic control system to redirect themeasurement beam in response to changes in at least one of angularorientation and position of the measurement object, wherein one of astage supporting the wafer and an illumination system producing theradiation includes the measurement object.
 68. A lithography method foruse in the fabrication of integrated circuits comprising: directinginput radiation through a mask to produce spatially patterned radiation;positioning the mask relative to the input radiation; measuring theposition of the mask relative to the input radiation using aninterferometry method; and imaging the spatially patterned radiationonto a wafer, wherein the interferometry method comprises: directing areference beam along a reference path and a measurement beam along ameasurement path contacting a measurement object; combining thereference and measurement beams to form overlapping exit reference andmeasurement beams, the overlapping exit measurement and reference beamsindicative of changes in a relative optical path length between thereference and measurement paths; and using an electronic control systemto redirect the measurement beam in response to changes in at least oneof angular orientation and position of the measurement object, whereinone of a stage supporting the mask and a illumination system providingthe input radiation include the measurement object.
 69. A lithographymethod comprising: positioning a first component of a lithography systemrelative to a second component of a lithography system to expose a waferto spatially patterned radiation; and measuring the position of thefirst component relative to the second component using an interferometrymethod comprising: directing a reference beam along a reference path anda measurement beam along a measurement path contacting the firstcomponent; combining the reference and measurement beams to formoverlapping exit reference and measurement beams, the overlapping exitmeasurement and reference beams indicative of changes in a relativeoptical path length between the reference and measurement paths; andusing an electronic control system to redirect the measurement beam inresponse to changes in angular orientation of the first component.
 70. Abeam writing method for use in fabricating a lithography mask, themethod comprising: directing a write beam to a substrate to pattern thesubstrate; positioning the substrate relative to the write beam; andmeasuring the position of the substrate relative to the write beam usingan interferometry method comprising: directing a reference beam along areference path and a measurement beam along a measurement pathcontacting a measurement object; combining the reference and measurementbeams to form overlapping exit reference and measurement beams, theoverlapping exit measurement and reference beams indicative of changesin a relative optical path length between the reference and measurementpaths; and using an electronic control system to redirect themeasurement beam in response to changes in angular orientation of themeasurement object.
 71. A distance measuring interferometry systemcomprising: an interferometer which during operation directs a referencebeam along a reference path and a measurement beam along a measurementpath contacting a measurement object and combines the reference andmeasurement beams to produce overlapping exit reference and measurementbeams, the overlapping exit reference and measurement beams indicativeof changes in a relative optical path length between the reference andmeasurement paths, the interferometer comprising a beam steeringassembly having a beam steering element and a positioning system toorient the beam steering element, the beam steering element having atleast two faces positioned to direct at least one of the reference andmeasurement beams, the at least one of the reference and measurementbeams contacting each of the two faces during propagation within theinterferometer; and a control circuit which during operation causes thepositioning system to reorient the beam steering element in response tochanges in at least one of angular orientation and position of themeasurement object.
 72. The system of claim 71, wherein during operationthe interferometer further separates an input beam into a pair ofspatially separated beams to define the reference and measurement beams.73. The system of claim 71, wherein during operation the interferometerreceives a pair of spatially separated input beams to define thereference and measurement beams.
 74. The system of claim 71, wherein theinterferometer is a single-pass interferometer in which the measurementbeam contacts the measurement object only a single time.
 75. The systemof claim 71, wherein during operation the control circuit causes thepositioning system to reorient the beam steering element so as tomaintain the exit reference and measurement beams substantially parallelto one another over a range of angular orientations of the measurementobject.
 76. The system of claim 71, wherein during operation the controlcircuit causes the positioning system to reorient the beam steeringelement to reduce changes in direction of the exit measurement beamcaused by changes in the angular orientation of the measurement object.77. The system of claim 71, wherein during operation the control circuitcauses the positioning system to reorient the beam steering element toreduce a transverse displacement between the exit reference andmeasurement beams caused by changes in the angular orientation of themeasurement object.
 78. The system of claim 71, wherein during operationthe control circuit causes the positioning system to reorient the beamsteering element to reduce a transverse displacement of the measurementbeam caused by changes in the angular orientation of the measurementobject.
 79. The system of claim 71, wherein during operation the controlcircuit causes the positioning system to reorient the beam steeringelement based on a signal generated from the exit measurement beam. 80.The system of claim 71, wherein during operation the control circuitcauses the positioning system to reorient the beam steering element suchthat the measurement beam contacts the measurement object atsubstantially normal incidence over a range of angular orientations ofthe measurement object.
 81. The system of claim 71, wherein duringoperation the control circuit causes the positioning system to reorientthe beam steering element to reduce a transverse displacement betweenthe exit reference and measurement beams caused by translations of themeasurement object.
 82. The system of claim 71, wherein during operationthe control circuit causes the positioning system to reorient the beamsteering element to reduce a transverse displacement of the measurementbeam caused by translations of the measurement object.
 83. The system ofclaim 71, wherein the measurement object is a plane mirror and duringoperation the measurement beam contacts the measurement object atnon-normal incidence and the control circuit causes the positioningsystem to reorient the beam steering element to reduce transversedisplacements between the exit reference and measurement beams caused bytranslations of the measurement object.
 84. The system of claim 71,further comprising a polarizer which during operation receives the exitreference and measurement beams and mixes polarizations of the exitreference and measurement beams to produce a mixed beam having a phaseindicative of the changes in the optical path length difference betweenthe reference and measurement paths.
 85. A lithography system forfabricating integrated circuits comprising first and second components,the first and second components being movable relative to each other,and the interferometry system of claim 71, wherein the first componentscomprises the measurement object and the interferometry system monitorsthe position of the first component relative to the second component.86. The interferometry system of claim 71, wherein the at least one ofthe reference and measurement beams is the measurement beam.
 87. Theinterferometry system of claim 86, wherein during operation theinterferometer directs the measurement beam to contact one of the twofaces of the beam steering element before the measurement beam contactsthe measurement object, and the other of the two faces of the beamsteering element after the measurement beam contacts the measurementobject.
 88. The distance measuring interferometry system of claim 71,wherein the beam steering element comprises a mirror on each of thefaces.
 89. A method for interferometry comprising: directing a referencebeam along a reference path and a measurement beam along a measurementpath contacting a measurement object and each of at least two faces of abeam steering element; combining the reference and measurement beams toform overlapping exit reference and measurement beams, the overlappingexit measurement and reference beams indicative of changes in a relativeoptical path length between the reference and measurement paths; andusing an electronic control system to cause the beam steering element toredirect the measurement beam in response to changes in at least one ofangular orientation and position of the measurement object.
 90. Theinterferometry method of claim 89, wherein the measurement beam isdirected to contact one of the two faces of the beam steering elementbefore contacting the measurement object, and the other of the two facesof the beam steering element after contacting the measurement object.91. The distance measuring interferometry system of claim 89, whereinthe beam steering element comprises a mirror on each of the faces.
 92. Adistance measuring interferometry system comprising: an interferometerwhich during operation directs a reference beam along a reference pathand a measurement beam along a measurement path contacting a measurementobject and combines the reference and measurement beams to produceoverlapping exit reference and measurement beams, the overlapping exitreference and measurement beams indicative of changes in a relativeoptical path length between the reference and measurement paths, theinterferometer comprising a beam steering assembly having a beamsteering element and a positioning system to orient the beam steeringelement, the beam steering element positioned to direct the referenceand measurement beams, the reference and measurement beams contactingthe beam steering element; and a control circuit which during operationcauses the positioning system to reorient the beam steering element inresponse to changes in at least one of angular orientation and positionof the measurement object, wherein the interferometer is a single-passinterferometer in which the measurement beam contacts the measurementobject only a single time.
 93. A distance measuring interferometrysystem comprising: an interferometer which during operation directs areference beam along a reference path and a measurement beam along ameasurement path contacting a measurement object and combines thereference and measurement beams to produce overlapping exit referenceand measurement beams, the overlapping exit reference and measurementbeams indicative of changes in a relative optical path length betweenthe reference and measurement paths, the interferometer comprising abeam steering assembly having a beam steering element and a positioningsystem to orient the beam steering element, the beam steering elementpositioned to direct the reference and measurement beams, the referenceand measurement beams contacting the beam steering element; and acontrol circuit which during operation causes the positioning system toreorient the beam steering element in response to changes in at leastone of angular orientation and position of the measurement object,wherein during operation the control circuit causes the positioningsystem to reorient the beam steering element based on a signal generatedfrom the exit measurement beam.
 94. A distance measuring interferometrysystem comprising: an interferometer which during operation directs areference beam along a reference path and a measurement beam along ameasurement path contacting a measurement object and combines thereference and measurement beams to produce overlapping exit referenceand measurement beams, the overlapping exit reference and measurementbeams indicative of changes in a relative optical path length betweenthe reference and measurement paths, the interferometer comprising abeam steering assembly having a beam steering element and a positioningsystem to orient the beam steering element, the beam steering elementpositioned to direct the reference and measurement beams, the referenceand measurement beams contacting the beam steering element; and acontrol circuit which during operation causes the positioning system toreorient the beam steering element in response to changes in at leastone of angular orientation and position of the measurement object,wherein during operation the interferometer further separates an inputbeam into a pair of spatially separated beams to define the referenceand measurement beams, and wherein the interferometer further comprisesa polarizing beam splitter that separates the input beam into thereference and measurement beams, and wherein the beam steering elementcomprises a beam steering mirror and the positioning system comprises atleast one transducer mounted to the beam steering mirror and operativeto adjust an orientation of the beam steering mirror in response to acontrol signal from the control circuit.